sipgate.co.uk is quite good for providing a free PSTN ->to-> SIP gateway, you have to buy credit for SIP->PSTN

you get a choice of geographic number (I have two accounts, one is for my rural/home area, one for nearest city) or the non-geo number (0845 etc).

one nice feature of sipgate is call forking; I can be logged in with n800 and also with sip phone (siemens cordless/DECT + voip thing) so both ring and the first to answer completes the call.

downside: their voicemail system sounds as if it were recorded by a prison guard!