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ONSIM vs Tamar Telecommunications: which is right for you?
Tamar Telecommunications is one of the most established names in UK virtual numbers - 20+ years trading, 25,000+ businesses, superb reviews. ONSIM competes at the same £5 entry point but takes a different route upward: instead of upgrading you into VoIP, ONSIM moves your landline number natively onto a SIM.
This page is a fair side-by-side. Pick the one that fits your business, not the louder marketing.
Side-by-side
All prices ex VAT. Tamar pricing checked July 2026 (virtual numbers from £5/mo, VoIP from £8/mo published on their site); verify live before deciding.
| ONSIM | Tamar Telecommunications | |
|---|---|---|
| What it is | Virtual landline (divert) + SIM-native landline | Virtual numbers / call forwarding + cloud VoIP |
| Entry price | £5/mo + VAT | From £5/mo (virtual numbers) |
| Upgrade path | £10/user/mo Landline SIM - number natively on SIM/eSIM | From ~£8/mo cloud VoIP - app or desk phones |
| Outbound calls show landline number | Yes - natively via Landline SIM, no app | Yes - via Tamar VoIP app/handsets |
| Requires app for outbound landline calls | No (SIM-native) | Yes (VoIP app or handset) |
| Free features at entry tier | Number + forwarding; extras mostly on Landline SIM | Hunt groups, timed diverts, greetings, missed-call alerts, call stats, WhatsApp Business |
| Number types | 01, 02, 03, 08 (650+ dial codes) | 01, 02, 0333, 0300, 0800 |
| Analogue-phone replacement box | Yes (Landline Replacement) | No - VoIP handsets instead |
| Contract | Monthly rolling, cancel anytime | 1-month rolling |
| Trading since | 2013 (SIM-native landline) | 20+ years (Kalnet4u Ltd, reg. 04655311) |
| Customers | 7,500+ | 25,000+ |
| Trustpilot | 4.6/5, 64 reviews | 4.8/5 (2,500+ reviews across platforms) |
When ONSIM is the better choice
- You want outbound calls to display your landline number with no app - the number lives on the SIM (Landline SIM, £10/user/mo)
- You never want call quality to depend on Wi-Fi, broadband or an app running in the background
- You want both the £5 divert tier and the £10 SIM-native tier under one provider, so you can upgrade without changing anything
- You need to keep analogue phones, alarms or lifts working after the 2027 PSTN switch-off without broadband (Landline Replacement)
When Tamar is the better choice
- You want the richest free-feature bundle at the £5 forwarding tier - hunt groups, timed diverts, greetings, missed-call alerts and call statistics included
- Your growth plan is a multi-user cloud VoIP system with desk phones and extensions
- You want a large, long-established provider - 20+ years, 25,000+ businesses, 4.8/5 reviews
- You want same-day number setup and highly rated UK phone support
More about Tamar at tamartelecommunications.co.uk.
Frequently Asked Questions
Common questions before choosing between the two.
What's the main difference between ONSIM and Tamar Telecommunications?
Both sell UK virtual landline numbers from £5/month that forward to your mobile - at entry level they are direct rivals. The difference is what you upgrade into. Tamar's next step is a cloud VoIP phone system (from around £8/month) using an app or desk phones. ONSIM's next step is the Landline SIM (£10/user/month) - the landline number moves natively onto a SIM/eSIM, so inbound and outbound calls work like normal mobile calls with no app, no VoIP and no broadband dependency.
How does pricing compare?
ONSIM Virtual Landline is £5/month + VAT with up to 2,500 forwarding minutes; ONSIM Landline SIM is £10/user/month + VAT with unlimited UK calls and SMS (fair use). Tamar's virtual number plans start from £5/month and their VoIP plans from £8/month as published in July 2026 - check tamartelecommunications.co.uk for live pricing and minute bundles before deciding.
Which has better reviews?
Both are strongly rated. Tamar has a 4.8/5 Trustpilot rating and cites over 2,500 reviews across platforms - a genuinely excellent, long-earned reputation. ONSIM has a 4.6/5 Trustpilot rating from 64 reviews. Tamar has been trading for over two decades and serves 25,000+ UK businesses; ONSIM has run SIM-native landline services since 2013 and serves 7,500+ customers.
Is ONSIM a good Tamar Telecom alternative?
It depends what you're moving for. If you want outbound calls to display your landline number natively - without a VoIP app - ONSIM's Landline SIM does something Tamar's forwarding plans don't. If you're happy with inbound forwarding and value free extras like hunt groups and timed diverts at the £5 tier, Tamar is a strong place to stay.
When is Tamar the better choice?
Pick Tamar if you want the most feature-rich £5 forwarding plan: they include hunt groups, timed diverts, welcome greetings, missed-call alerts, call statistics, holiday settings and WhatsApp Business as free features at entry level, with same-day setup and well-regarded Devon-based support. Also pick Tamar if your growth plan is a multi-user cloud VoIP system with desk phones - that's their upgrade path and they do it well.
When is ONSIM the better choice?
Pick ONSIM if you want the landline number to live natively on a SIM/eSIM (Landline SIM), so outbound calls show your landline number without an app; if you want call quality that never depends on Wi-Fi or an app being awake; or if you need a plug-in Landline Replacement box to keep analogue phones, alarms or lifts working after the January 2027 PSTN switch-off without broadband.
Can I port my number between Tamar and ONSIM?
Yes, in both directions - UK number portability is mandatory and both providers support porting for geographic, 0333 and 0800 numbers. ONSIM's port-in typically completes in 10-14 working days; port-in is included on Landline SIM (one-off £20+VAT on Virtual Landline) and the port-out fee is £20+VAT.
Do both work without broadband?
At the forwarding tier, yes - both route calls over the phone network to your mobile. Beyond that they diverge: Tamar's upgrade path (VoIP) moves calls onto the internet, while ONSIM's upgrade path (Landline SIM) keeps calls on the mobile network natively, so a broadband outage never touches your calls.