Thanks, I didn't know that. I remembered there being some issue with local #'s in the UK used for VOIP, but I assumed it was due to taxing calls to some extent.
Certain numbers like 0845 and 0870 - are supposed to be charged at 'local' and 'national' rates from all locations in the country. In reality, this is only true when using BT to dial. These numbers are 'shared revenue', in that the receiving customer gets cash for receiving calls (or in many cases, they get their service rental paid). Most providers (apart from BT), including mobile operators, charge these at ludicrously expensive rates.
This issue is supposed to be a lot simpler with 05 numbers, as they don't make provision for a 'kickback', although the VOIP provider can charge the outgoing telco a slightly higher incoming call fee. BT charge these at 'national' rate (and they come out of inclusive minutes, etc.). Many other providers also charge these at 'national' rate on their regular tariffs, but this may not be universally the case. Unfortunately, the complexity of telephone charging in the UK is preposterous, and varies widely among providers.
For total transparency, there are 03 numbers - these are available for VOIP and other business services. These are guaranteed to be charged at the normal 'national landline' rate on all providers, including mobile, and for incoming international calls. However, these command a premium price, as there is no income from cross-connection charges.