Beware that "low cost provider" may not just reject the call, which you can recover from, but can mess you up in a large number of ways. Some of which you can try to work around (debugging + time), some of which you may not be able to figure out.

Not properly passing along DTMF
very long PDD
FAS fake answers
Returning "USER_BUSY" when it's not actually busy
Returning "NORMAL_TEMPORARY_FAILURE" when it's actually busy
poor audio quality
Issues with proxies/re-invite/switching codecs, etc.
.. and I'm sure many more.

... Just some of the many issues I've run into when using multiple carriers and terminating to countries around the world.

-Avi Marcus

BestFone