I'm only a light mobile phone user, and changed from contract to PAYG in '06 having been with Vodaphone for about 12 years. And with the same number. Before that I was on Cellnet analogue.
I only used my mobile a few times last week - mainly for texts. Both sending and receiving. On Friday evening when going out and actually looking at it, got the message 'SIM card registration failed'
Phoned the helpline from my landline. They said that number doesn't belong to me, but to a contract customer. On giving them details of who I'd texted, they admitted there was a problem, but I'd have to go to a Vodaphone shop to get it sorted.
Which I did on Monday. Systems down, so nothing they could do. Come back later.
So I did today. At about 11:45. Left at 13:55, still not resolved. The assistant was as helpful as could be - despite a busy shop.
They think it may be an attempted fraud. Normally, they let a number lie dormant for a year or so before re-using it.
Would be a real PITA to have to change my number.