From the FAQ

Will the Jolla Phone work outside Europe, can I use it e.g. in the U.S.? Will the Jolla Phone work outside Europe, can I use it e.g. in the U.S.?

Yes, we will design the cellular band configuration to enable global travelling as much as possible, including e.g. roaming in the U.S. carrier networks.

Edit: Wrong reply, wanted to add that to the comment about US "goofy bands"