...as Boule’tard said:
I too have an old but, very capable (and kinda bricky) Nokia N95
with a tmobile pay-as-you-go sim card, works fine for me
and $20 credit last forever,
specially if you call no one
and just let others call you.
BTW, it's VERY important to "reprogram" your web-browsing
settings, this way your phone cannot "decide" to suddenly
connect to your service provider's signal and drain your credit;
now you can only connect to wifi hotspots or your home network.