In general all Voice Providers will work with Elastix providing they support SIP or IAX protocol.
If they support Asterisk, or Trixbox, your Elastix system will work with their services.
In general, your Elastix, in terms of setup of IAX and SIP trunks, is generally the same as Trixbox.
The best thing is find a Voice Provider that supports IAX or SIP, search google for Trixbox/Asterisk settings and that particular Voice provider, and generally you will find someone else has published a config.
Or choose a provider, and tell us that provider on this forum, and you will find that many will be willing to assist you.
Regards
Bob