I have some clients that do not check their email often.
I always manually send them an SMS using SMS global. They then pay a few hours later.
I think it would be a great idea to implant the use of SMS gateways into WHMCS without the use of addon modules.
We already have the client's mobile numbers data.
So an example would be that only when an invoice reminder email is sent, an SMS is also sent out notifying the client.