Still, Verizon has posted a job for a "Global LTE Roaming Manager - Canadian and Western European Countries", indicating that at some point we will be globetrotting between LTE networks. And not in a distant future at that - the position is looking at a 3-5 years timeframe to build carrier relationships and figure out the interoperability issues.
And there are many of those, apart from the MHz conundrum - the other carrier might be using IMS with VoLTE for the talking part, or switch to HSPA+ when a phone call has to be made like AT&T does, or just us Skype, and it all has to somehow work, so the LTE Roaming Manager better be well paid. Read the job description below for the nitty-gritty:
source: Verizon via IntoMobile