If you used one of those SIM cards here, you'd be paying roaming tariffs

Some VoIP providers have a bit lower charges to UK mobiles, and if you wanted a landline diverted to your mobile, this would be the way to do it, but you'd still have the call forwarding to pay for.

Callback services also mainly use VoIP and can be used where a lower tariff than the SIM's own can be found, often when roaming abroad and wanting to call home, though there are an increasing number of mvno or different branded SIMs with low tariffs for international calls, even here.

Overall, it is difficult to avoid extra costs of mobile over landlines, unless you have a contract with inclusive minutes that can be used to various callthough numbers.

But there are VoIP clients arriving that can use wifi access in PDAs or smartphones, and rumours of some that use VoIP over 3g, but even here there are access or bandwidth costs in many cases.