He says he also has this problem after he flys someplace
That'll be your main cause. When a device roams and then comes back to its home network it needs to log on to several separate segments of the network - voice, data, SMS, etc. Ocassionally this doesn't all go to plan.
What I do is get the user to perform a manual roam to a different network, then do a manual roam back on to the network they are supposed to use. This basically de-registers the SIM from the home network and then re-establishes it again. I'd bet you a weeks wages this fixes it every time.