Opt-in and Opt-out Feature

Gentlemen,

Providing that Opt-in and Opt-out is a Bulk SMS Best Practice feature, we would like to know if:

01 - Is this featured available on PlaySMS?
02 - How to enable it?

Thank you!

Regards,
Antonio Santos

Currently this feature is not available in playSMS

Just in case someone willing to write it, what are the requirements ?

anton