I had a mobile SIM with Vodafone, purely for internet use away from home, used very infrequently. When I went to top it up I was told that, under new government regulations, unused SIMs expire after 3 months, the reason being the shortage of available numbers; destroying the SIM releases the number for others. I was given a new SIM free of charge but the message is: even if you have a phone for emergency use only (as I do) you may have to use it every 3 months or it may become useless (and you'll lose any credit on it).