Our new Siemens phone arrived yesterday and after working to solve all the problems that we had with our friend Michael's phone, I was able to set it up quickly.
There are configuration settings for a range of VoIP providers stored in the phone but not ones for VoIPtalk. The best we could do was to apply the settings for a different provider and then change the details to make a connection to VoIPtalk. There are obviously some subtle differences between providers though.
What we found with Michael's phone and initially with mine was that we could make calls and speak to the person at the other end and they could hear us. When people tried to return the call though the phone would ring but all they got at their end was a number unavailable tone and at our end nothing.
I went through the settings again on our phone changing them and then changing them back and suddenly bingo it worked. Now I only tried it once by phoning our number from my mobile so we might not be out of the woods yet but at least we know that it can work.