Hi to all, I've just received a mail from customer service of diamondcard.us:
Hello Ercole,
We have added the ability to sign-up and recharge with Bitcoins. Login to your account, click recharge and choose Bitcoins as your payment method. The minimum amount for payment is $1.
Let us know how it works.
Sincerely,
Diamondcard.us Support Team
Diamondcard is a VOIP, Virtual PBX and other phone related service provider.