I might use one of Vonage, Google Voice Callback (with a Callcentric SIP account as the callback number) Localphone or GrooveIP.
This worked fine on my last trip in August last year on my GN.
This time I found I could not make any SIP calls over the 3G network (no LTE because my phone is the international version of the N5). Calls would be placed only to be greeted with silence and GV callback would never call me back. The only method that worked was GrooveIP but apparently that's going to stop soon because of the way Google manages some API with Google Voice.
All the processes worked fine over WiFi.
Just to check I tried my daughter's N4 on the same plan and she could make all those calls without any problem.
I see TMO has blocked tethering on the N5 which can be overcome through Root and modifying some file but does anybody know if TMO has also blocked SIP calls on the N5 also?
Might have to take my GN with me next time as my US phone and keep the N5 with the home SIM card in it.