You can change the free number every time
I have the T-Mobile on the Pay and Go Best Friend tariff and I think it’s great. It suits me perfectly. If you have lot of friends or any other land line number that you ring regularly, then this tariff is ideal. For a credit of a minimum of £15 every 60 days you can make up to 500 calls per 30 days (a bit confusing I know) to that free number. You can change the free number every time you top up your phone, and you can credit it with as little as £10 to change that free number. Call time vouchers can be obtained in £10, £15 and £20 denominations and can be got virtually anywhere. I never have too much difficulty finding somewhere with a stock.
