Good catch! Looks like you've stumbled on the new Cobb PD Precinct 2 ISSI roaming on the Fulton system. This allows Cobb PD precinct 2 portables to roam onto the Fulton system and bring their Precinct 2 traffic with them for so long as they need the link (e.g. while they are at a location with insufficient Cobb system coverage but adequate Fulton system coverage). These links are not full time. They become active for a particular transmission that requires ISSI roaming to complete, and stay active only as long as roaming is required for adequate communications. Also, it appears the 'foreign talk group' number on the Fulton system is dynamically assigned as needed on a per-transmission basis. I've been listening this morning and am sporadically hearing Cobb PD precinct 2 on Fulton 65398, 65396 and several other nearby Fulton talk group numbers (confirmed via a 2nd radio on the Cobb simulcast). Of course, most of the time I hear nothing on the Fulton system when Cobb Precinct 2 is active, because no Cobb subscriber radio is needing the Fulton ISSI roaming at that moment.
It appears the North Fulton system has ISSI roaming with Cobb Precinct 4 too. Precinct 4 can sporadically be heard on random NFRSSA talk groups too.
Obviously this is all a big challenge for scanners given the dynamically-assigned nature of the foreign talk group number.
55023 is a much easier answer. That's Fulton County 911 Training. I bet you were hearing dispatcher training.