I was able to solve this here guys! It was a matter of configuration, for some reason parameter append_fromtag on rr module is being set by default in original kamailio.cfg to 0 and this complicates everything. It also needed me to load module uac.so and add outbound.so module to the configuration file! It now works like a charm, so I'm putting what I did here so that other people with same problem can find the solution here