all of the providers should be able to top up online. I did it with my One2Free card over the last year here:
http://www.one2free.com/jsp/prep ... recharge_method.jsp
make sure you do it before it expires (6 months, as mentioned above).
Here in the USA, T-Mobile has a pretty sweet deal - buy a 1000 minute card for $100, then once a year you simply need to top up for only $10 (adds 100 minutes) to retain your phone number. This is off topic, but will be of use to some of you: to make myself totally anonymous, 1) I bought the T-Moble prepaid SIM by cash at a Chinatown in Chicago. You can also do this on EBay, or even at the T-Mobile store but it will be more expensive. 2) paid for the 1000 minute option, by cash, at the T-Mobile store. When you set up, they don't need any of your personal info - just a name and a PIN code and a birthday date for secondary verification. 3) My top ups are by cash at the T-Mobile store as well. 4) if you don't have a lot of contacts, or any contacts, you can and should switch phone numbers every 6 months or so. You can even use different area codes around the country. T-Mobile is the way to go, since they and AT&T are the two main national providers that are on a GSM network. (Sprint, Verizon and Nextel are not).