This method is available in Control Panel -> $ -> Recharge of personal account:

It is a quick and easy way to add funds to your personal account. Select you country, mobile operator and the amount you want to pay, and send the text message with the code the system will give you, to the given number.

Pay attention to some limitations:

1. You cannot use more than 3 different phones to recharge one and the same account.
2. You cannot credit more than $15 and send more than 7 messages per day. These numbers are counted for all your uCoz accounts/websites.