Mobile phone systems for accountancy firms
YOU SAID FILE IT THAT WAY.
THE CLIENT SAYS THEY NEVER DID.
Filing instructions, fee agreements, "just put it through the company" - the conversations that matter happen by phone, on a partner's personal mobile. Months later, when HMRC asks questions, the client remembers that call very differently.
ONSIM puts a firm-owned number on every accountant's handset, natively ON-A-SIM. Optional network-level recording captures calls and texts as a contemporaneous record of what was actually said, and attempted-call logs prove you chased the client for their records - even when they did not pick up.
Hunt groups and timed routing keep the January call crush answered, and when someone leaves, their number stays with the firm.
The advice is given by phone. Keep a record of it.
Tailored Phone Solutions for Accountancy Firms
Key Features Built for the Practice:
A practice phone system that lives on the handsets your team already carries. No apps, no desk phones, no VoIP - the firm's numbers, on the team's SIMs, with routing and records behind them.
Numbers That Belong to the Firm
Every accountant's business number is issued to the firm, not the individual. When a partner or manager moves on, reassign the number in minutes - to a replacement's SIM, a colleague's SIM, a hunt group or an IVR menu, with incoming texts delivered to email, no SIM required. The client relationships stay with your practice.
Contemporaneous Evidence of Advice
On-network call recording (an optional paid add-on) captures what was said about the filing, the treatment and the fee - at network level, with nothing for anyone to remember to press. With recording enabled, SMS is captured alongside calls, so the "just do it that way" text is on the record too.
Proof You Chased the Records
Real-time call logs include attempted calls - outbound calls that rang unanswered. When a client blames a late filing on the firm never chasing them, you have a timestamped record of every attempt made before the deadline. Most mobile networks cannot show you this at all.
Built for January
Hunt groups ring the whole team so a panicking client on 30 January always reaches a human, not one person's voicemail. Time-of-day routing follows your deadline-season rota, failover passes unanswered calls to a colleague, and you change it all yourself in the dashboard.
Personal Numbers Stay Private
The firm's number rings on each accountant's existing handset - SIM or eSIM, no app, no VoIP. Clients save a firm number, not a partner's personal mobile, so evenings and holidays stay personal and the client book stays professional.
Texts on the Business Number Too
Your team texts clients from the firm's number, not their personal one - reminders, record requests, approval nudges. Incoming texts can also be delivered to email, so a message to a number with no active SIM behind it still reaches the practice.
"You told me to file it that way."
Every practice knows this call. A treatment gets queried, a penalty lands, and the client's memory of the conversation is suddenly very specific: you told them it was fine, you never warned them, you said the deadline could slip. The instruction was given by phone - on a partner's personal mobile - and there is no engagement letter clause, file note or email that captures what was actually said in that ten-minute call.
The file note written afterwards is your word against theirs. A recording is not. With ONSIM's optional on-network recording, calls and texts on the firm's numbers are captured at network level - nothing to press, no app to open - giving you a contemporaneous record of the instruction, the caveats and the advice. And attempted-call logs show every time you rang the client for their records before the deadline, whether or not they answered.
There is a regulatory angle too. Accountancy service providers fall within the scope of the Money Laundering Regulations 2017, which carry record-keeping duties, and professional bodies generally expect engagements and significant advice to be documented. A network-level record of client calls and texts can support that documentation trail - though what you must keep, and for how long, is a question for your own compliance advisers. This is not legal or regulatory advice. Existing numbers - including mobile numbers clients already have saved - can be ported in and recorded and managed the same way.
What our customers say.
" One of the best services I have ever received! "
Mubasher
Small business owner
" Amazing company the service is second to none! Support staff are great and have helped me so much. Really polite and helpful. "
Smith & Sullivan Ltd
Small business owner
" My name is John Morrison I have two businesses and the staff at ONSIM were brilliant.....instant replies and very polite couldn't ask for more from them. Thanks for everything "
John Morrison
Small business owner
Still have doubts?
Our partners already use their personal mobiles with clients. How do we unpick that?
Two routes, and most firms use both. New joiners get a firm-owned number on a Landline SIM (physical or eSIM) from day one, so clients only ever save a firm number. For numbers already in circulation, if the number is a business asset you control, port it to ONSIM and it becomes reassignable like any other - clients keep calling the number they have saved, and the firm decides whose handset it rings.
Can we port in existing mobile numbers our accountants already use?
Yes. Existing UK mobile numbers can be ported in and managed exactly like any ONSIM number - reassignable, routable, and recordable if you enable the recording add-on. Port-in is included on the Landline SIM and a one-off £20 + VAT on the Virtual Landline. Clients notice nothing: the number they have saved just keeps working, now under the firm's control.
What happens to a number when a partner or manager leaves?
You reassign it in the dashboard, typically within minutes - and it does not need a SIM waiting for it. Point calls at a successor's SIM or eSIM, a colleague's SIM, a hunt group of the team or an IVR menu, and have incoming text messages delivered to email while you recruit. The number never goes dark, the leaver's client book keeps reaching your practice, and their personal number and personal life are untouched.
How does call recording help with "you told me to file it that way" disputes?
On-network recording (an optional paid add-on) captures calls - and, with recording enabled, texts - on the firm's numbers at network level, with nothing to press and no app to open. When a client disputes what was advised or instructed by phone, you have a contemporaneous record rather than a file note written after the event. It is evidence of what was said; whether and how you rely on it is for your own advisers.
Do we have to tell clients calls are recorded?
If you enable recording you are responsible for using it lawfully - in practice UK businesses typically inform callers, for example with a short announcement or a line in your engagement terms. Recording is off by default and only enabled if you opt in. See mobile call & SMS recording. This is not legal advice.
Does this help with Money Laundering Regulations record-keeping?
Accountancy service providers are within the scope of the Money Laundering Regulations 2017, which include record-keeping duties, and professional bodies generally expect significant advice and instructions to be documented. A network-level record of calls and texts can support that documentation trail, but what the regulations and your professional body require of your firm is a matter for your own compliance advisers - this is not legal or regulatory advice.
Can we prove we chased a client for their records before a deadline?
Yes. ONSIM's real-time call logs include attempted calls - outbound calls that rang unanswered - timestamped against the number dialled. When a client blames a late filing or a missed deadline on the firm never chasing them, the record of every attempt exists whether or not the call connected. Read why other networks don't log attempted calls.
How do we survive the January self-assessment call crush?
Hunt groups, included on 2+ user accounts, ring the whole team simultaneously or in sequence so a client with a 31 January deadline always reaches a human. Failover passes unanswered calls to a colleague, time-of-day routing follows your deadline-season rota (including weekend cover), and holiday routing handles February when everyone finally takes leave. You change all of it yourself in the dashboard.
We have more than one office - can each keep its own local number?
Yes. Each office keeps or picks its own local number - see local numbers for every UK area code - with its own routing and hunt group, all under one account. A SIM can carry more than one number, so a partner covering two offices displays whichever office they are calling for.
What does it cost, and does the PSTN switch-off affect it?
The Landline SIM is £10/user/month + VAT with unlimited UK calls and SMS, hunt group included on 2+ user accounts, and UK data bundles from £2. A Virtual Landline at £5/month + VAT suits a single practice number forwarded to a mobile. Recording is a paid add-on - most SMB customers pay £20/user/month + VAT all-in for a recorded user - and AI Insights transcription is a further opt-in add-on. Everything runs at mobile network level, not over your broadband, so the PSTN switch-off does not affect it - and your existing landline number can be ported in before your copper line is retired. Monthly rolling, cancel any time.
Flexible pricing plans
Virtual Landline
Start simple - inbound calls forwarded to your mobile
- Inbound calls forwarded to your mobile
- No SIM swap needed
- Pick new landline number or use existing
- Up to 2,500 forwarding minutes/month
- No app, no VoIP
- Cancel any time
Landline SIM
Firm-owned numbers on every accountant's handset
- UNLIMITED UK Calls & SMS**
- Numbers owned by the firm, reassignable in minutes
- Hunt groups, failover and time-of-day routing
- Standard or eSIM
- UK Data Bundles (2GB,4GB,10GB,20GB,40GB @ £2,£4,£6,£8,£10)
- Pick new numbers or port existing
- Cancel any time
- Trusted by 7,500+ UK businesses
Want to speak to our sales team? Call us now on +443338804008
All prices shown are ex VAT
**UK Mins are outbound calls to major (O2, Vodafone, EE and Three) UK mobiles (07), landlines (starting 01, 02, 03) and freephone numbers (0800, 0808). Unlimited call and SMS allowances are subject to our fair usage policy (5,000 mins and 1,000 SMS). Usage outside of these allowances are charged at 5p/min & 5p/sms. By default we only allow calling to inclusive numbers (excluding non-standard 07 numbers) to avoid incurring additional charges. You can request that these restrictions are removed at our sole discretion.