as a former 18 year resident of Mexico that started and ran a VoIP business for many years.
Telmex does not block VoIP, nor do I believe they ever did. . This was a myth that appeared long ago.
My recommendation if you have a place to leave the Obi connected to INternet in the USA .
1) get a dynamc Host Name for free at
https://www.duckdns.org/ 2) Install asterisk on a raspberry Pi and connect it to the USA location*
3) configure raspberry Pi to dynamic hostname
4) use the guide at nerd vittles to configure asterisk to make calls by way of Obi
http://nerdvittles.com/?p=258465) Get GS wave or other softphone and register it to your asterisk by way of the dynamic hostname.
* You can use a cloud based installation of asterisk or PBXes to do a similar thing as you would do with asterisk on a raspberry pi, but you will still need a dynamic hostname.
I can help you with this if you like
Mark
+1 360 553 0134