it is possible that the phone has lost its link with the network - we used to always advise clients to do a manual roam. You don't say what mobile you are using, but if you look in the settings, network, click on manual and the phone will look for networks, click onto any other network that comes up and allow it to say declined or whatever, then click onto VF. if this doesnt remedy the problem, would suggest you then call VF cust services as they can do other diagnostics and run through settings and the like.