FOP2 listens to SIPPeer events advertised by AMI. So, if you have those events properly set (And sip peers using qualify=yes), then you won't have issues. If you do not have qualify=yes or for some reason events are not being received, then you might have some. Take into account that PJSIP qualify status was added on a very recent FOP2 version (2.31), so if you use PJSIP you must use the latest FOP2.

Also, if you use sip peers using Asterisk realtime, there is a known issue with it , as the sip stack does not know about the initial state of peers, and it has some issues with keeping that up, you must be sure to cache realtime friends in your sip configuration, also, as the initial state is not know, you might want to use StartNotRegistered as true in the FOP2 General settings, so all extensions start as not registered from the start and light up when a presence event is received.