VoIP app compatible with the n95 (though I suspect that Orange will have made sure this stuff won't work).

Jon’s Phone Tool (JPT) uses a Bluetooth mobile phone, your Mac’s modem, or speakers to dial the phone. JPT can also make calls via Vonage, CallVantage, Skype, Ovolab Phlink,

Mac/PC, free calls to fellow users (ie the same as using iChatAV. Cheap-ish calls to proper telephones, too.

British Telecom appears to be explicitly blocking VoIP for their DSL subscribers.