My first guess would be network issue, this has been happening a bit lately and I believe its because of a number of new 4g towers rumored to be going online next month. Here is the list of reported up coming 4g upgrades..
I can definitely understand how frustrating it can not having a signal when you are traveling. I have gone on trips and not been able to get a connection also. Are you still having connectivity issues? Have you tried to enable/disable airplane mode on the phone? If you are still having connectivity problems, please PM your name and number and I would be more then happy to review your account to see how I can resolve this for you. I hope this helps. Thanks.