Your Telephony balance can be topped up in Telephony > Connection section:
You can buy 10$ or 30$ Telephony credit pack.
The amount you first credit to your account will be the amount that will be topped up automatically (e.g. if you buy $10.00 of Bitrix24 Telephony credit, your account is re-charged with $10.00 when it falls below the threshold). You can check Bitrix24 telephony prices here.
You can purchase telephony credits with your credit card or via Paypal: