I used the built-in sipgate settings and easily set up incoming and outgoing calls and all was well but I didn't want to commit to a fixed monthly amount, I wanted a pay as you go service. None of the currently installed providers seemed to do this (although I didn't go through the list exhaustively).
I found a US-based provder called voipvoip which seemed to work OK for North American calls but they claimed I needed the G729 codec installed to call UK numbers. Next I tried a UK-based SIP provider called voiptalk.org which was cheap and I could direct dial calls from LMCE to all the numbers I wanted, and they only have a GBP 5.00 minimum so there isn't a big commitment and they provide an 0843 incoming number free and split the revenue from it with you.
I found that I could easily test new providers (at least for outgoing calls) by just choosing the sipgate settings on the Phone Lines wizard and filling in the new SIP provider's hostname, account name and password. But now that I found a decent PAYG SIP provider I thought I would follow the instructions to properly add new SIP providers (http://wiki.linuxmce.org/index.php/How_to_properly_add_support_for_SIP_providers
). I used sipgate's entry as a template, duly changed all occurence of sipgate to voiptalk. It all seemed to work fine, the new provider came up in the wizard's list of providers, I filled in the host, username and password (which is what I did using the sipgate hack above) and clicked submit, but the list of phone lines appeared empty. So I've gone back to my hack of choosing sipgate as the provider but filling in voiptalk.org's details which seems to work. Does anyone have an idea why I can't seem to add a new provider?