The Facebook integration sends the call to facebbok as “offline other converions”. However, it’s possible to send offline standard events. It would make more sense to send the call as “offline lead” or “offline contact” events (or let us choose the type of standard event). When we create a Facebook campaign, we have to set a conversion event type and Facebook choose this event type to optimize the campaign. We can’t use “offline other conversions”, so we can’t use FB’s machine learning with our past call data. If the events were from the “lead” or “contact” standard event type, FB would take them in consideration for its campaign optimisation.
It’s possible to create a custom conversion and use this as a campaign objective, but as it’s explained here (Facebook Custom Conversions + Offline Event Sets), this means that we can only use one type of conversion (just the form or just the phone call)