As a reminder AT&T prepaid 'GoPhone' has 25cents a minute rates from Mexico [and 25cent SMS send/20cents receive]. That SIM would only work in Mexico so YMWV.

I am quite pleased with my Tru SIMs. If the rates are OK with you and it works where you are going that is great. Makes a good secondary US SIM also. About 10 months ago I was on a Mexico cruise and was able to receive SMS on the ship at sea with my Tru SIM. [free rreceive]. I was too cheap to test what its per minute call rates and SMS send rates were. You could test them!

On my single trip on a cruise ship I forwarded all my incoming calls to voicemail leaving a message telling callers to text me or leave a numeric page. I never found it necessary to place or receive calls at sea.

On my Regent Baltic cruise, I used straight SIP and it worked. Given what they charge (by the minute) for cruise ship internet, I'm shocked that they block VOIP. Oh well, anyway to gouge you

I found IAX2 is blocked less than Skype/SIP and fring. TMobile USA's UMA protocol is very hard to distinguish if IPSEC. It is a subspecies of IPSEC and looks similar to a commercial VPN. There are also very few subscribers so it lacks the critical mass needed for the blocking technology companies to invest much time in. If you have a iPhone, VOIPBuster has a VOIP app designed to work in the Middle East. VBuzzer has an Android App designed for the Middle East. SIP Droid on an Android running through a free VPN courtesy of PBXES.com works nicely with a free Google Voice account for free calls.