Hi!
For VoIP.ms they just seem not to fill in the register string...
For Flowroute it's a little more complicated, they put what they call their tech prefix followed by a star in the outbound dial prefix.
The other two I have not found the procedure yet but I assume it's similar...
I would try the way VoIP.ms does it, fill in everything as usual just don't provider a register string...
Good luck and have a nice day!
Nick