At retail stores i ask if they can charge the card amount first and then pay the rest separately.

Sorry for the late answer, but one of the reasons I've thought about registering a company is just so I can use up my own mostly-empty prepaids. Doing it at a store is another decent answer, but I always feel bad if I take too long at a checkout.

Hmm. I wonder if it would work on a self-checkout somehow.

I use Amazon to convert them fully into Amazon gift cards. Probably won't help with the mobile phone bill but it does empty the pp card fully and without fees.