Prepaid phones won't work. I got mine working after having the phone unlocked and then using a Roam Mobility SIM.
Monthly plans will work. Is it only for emergencies or an occassional call? If you are planning moderate use, consider a roaming plan.
The other thing to keep in mind is that if you use the Telus Easy Roam you are using your normal Canadian phone number. Therefore, calls/texts to family members back in Canada will not be any additional charges to your friends and family members. I'm not sure how the Roam Mobility works as I am an expert in only Telus products but, I would assume they give you a USA number? Maybe not, might want to check that in the FAQs. If they give you a USA number then your friends and family members back in Canada will incurr pay per use charges for texting/calling an American number. A good alternative a lot of my customers like is that one person does the easy roam for $7 per day to have their Canadian plan transfer over and their significant other uses the smaller prepackaged 30 day $25 combo pass. All things data related, they use in the easy roam phone and the other phone is just so that person can also be contacted or do the occasional text. The $25 pass includes 50 minutes calling, unlimited incoming text, 150 outgoing text and 50mb of data. All food for thought! You can add these yourself on Telus My account, chatting in or calling in at *611! If you do choose a Telus roaming pass for only one device make sure you put the other one on airplane mode so they don't accidentally get charges. Enjoy your trip!