I've been there on base at GTMO and the answer is yes and no. The ISP there SCSI is pretty much as terrible as they come. They do block SIP ports on purpose, however it will work at the coffee shop and on the MWR network if you take it with you and tether it. The wife and I used teamspeak to talk with each other :-).
Sam
Update, I talked to a buddy still on island and he *THINKS* that it will work on the Phoenix cable internet which is also available but not on the SCSI DSL. Also he confirmed SIP does work on MWR networks. The fiber isn't in place yet but is coming eventually for now it's still all via satellite.