Thanks for responding so quickly. Voipwise have not responded to my questions (and I don't think they will), and I don't think my handset is the problem, as I get the same results with my headset.
My Sip provider, Voipwise is offering free phone calls to UK landlines, which is correct when I use their softphone. When I place the same call through Express Talk I get charged. Any ideas how I can configure Express Talk to take advantage of the free calls.