Apologies if you already know this but I've been trying to get this sorted for a while now.
We all know that Truphone has a very handy wizard that enables you to make VOIP calls via a suitable WiFi spot. And we all know that Truphone doesn't support non-WiFi use, though it is rumoured possible, with several reports of success. However, trying to alter the Truphone wizard to create a suitable SIP setting is a pain and not guaranteed to work. (well my attempts were very hit and miss).
Well after much frustration and digging around with settings files, I had look at Fring, as this was reported to be an nice and easy tool for Skype users, with the added benefit of allowing one "Other" SIP configuration. And low and behold, configuring Fring to use Truphone bypasses the Truphone wizard (and with it the requirement to be in a WiFi AP) and simply dials the number via VOIP. Sorted.
The tricky part is actually configuring Fring correctly, as it needs a username, password and proxy. And these aren't obvious. The easiest way to find these is (1) Tools>Settings>Connection>SIP> select a Truphone profile, Options>Edit, scroll down to Proxy Server, Options>Change and scroll down to find your username(a string of 7 numbers in my case). Then copy the truphone.set (settings) file from your phone to your PC and open it in Notepad. Find your username by searching for the string. To the right you see another string of 9 letters and 1 number (well that's what it was in my case) and to the right of that, truphone.com. This string is used in as your password and for Proxy Server just use truphone.com.
Then just follow the instructions in Fring to add a VOIP service (Options>Settings>Configure services)