You can purchase credit from
https://secure.verivox.de/cards/index.asp?Lang=eng and pay by credit card.
But why do you still use this rip-off? Check out
solomo pro, which is always way cheaper than CallYa. Especially when calling abroad (USA for € 0.09/min vs. € 1.99/min) and while staying abroad, as they've good roaming rates - incoming roaming calls cost € 0.10-€0.15 in most European countries, while Vodafone charges twice as much.
The only disadvantage of solomo is, that they do not ship their cards to the US, so you need an European address for shipping. You can recharge the solomo pro SIM online by credit card and they also support automatic recharging, if you wish.