using custom Email to SMS Gateway

Hi, we would like to begin to use some Email to SMS Functionality within Password Manager, so send a OTP to our users to unlock/reset their accounts, rather than the usual Q&A scenario.

Is there any documentation about this? I have seen some comments on the forums, but nothing specific, nor much in the way of documentation on the Custom Activities and how they could be used.

Am I missing the documentation, or are there any examples anywhere for use of SMS Custom Gateways with the product?

Kind Regards