My girlfriend has an Xperia S, and she's now in Cuba. She has the roaming on, but she is unable to send messages or do phonecalls, cause she gets the message "IMSI unkwon in VLR error MM#4 ."
Depending on her operator she may need to manually select which network to use in that country. She can do that from Settings -> More -> Mobile networks -> Network operators. If that doesn't help, she needs to contact the operator she's using to have this problem resolved.
Had this same issue on my Xperia Z today.... I was not roaming on a different operators netowrking , I was in my home country on my normal operators network. First time occurance, although I do have problems with my celluar networks drops out completely from time to time.
