Port the number to AAISP if you can, I think its worth £1.44//month.
I'm actually doing the opposite, using VoIP more now as I have no need for an expensive mobile contract and just make one call a month to keep it active on a PAYG plan. Of course most people aren't always at home, but I think a lot of people should re-assess the cost of their mobile contracts as they might be overpaying.