With SMS Gateway module you can use one of built-in HostBill Notification modules (including Nexmo, Clickatell, Twilio) as a back-end to create and sell pre-paid or pay-as-you-go SMS products. The administrator can limit total number of sms to sent in month. The client can send text messages directly from client portal using User API module. Sent messages history can be browsed (and deleted if needed) from the client portal. The client can also get access to metered SMS billing usage.