This is pretty advanced for a beginner and there could be unexpected side effects. You can’t monitor and step down that 12v down to 3.5v without causing changes on that original line. You might end up messing with that original 12v enough to make the autodialer no longer work. Not a guarantee but it could be a problem.
You will probably want to search around on sites like Instructables and the like for the electronics part. Maybe post to an ESP forum. We have a lot of DIYers on this forum but they do not make up the majority of OH users.
The OH side it pretty easy. MQTT Binding (v1.11) Getting Started 101 will get you started on the OH side of things.