@jbenjamin,
Never fear help is here! Being able to use your phone while traveling is definitely important!
The HTC One is a 4G device and all our 4G device are unlocked to use with local carriers in other countries. You need to reach out to the local carrier for assistance in activating the phone with the local carrier. We are unable to troubleshoot the phone using another carrier's SIM card.
JohnB_VZW
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