This has been discussed previously. During the OpenSky years PEMA provided control stations to every county 911 center (as well as Pittsburgh, Allentown, and Bethlehem PSAPs). How the counties handled these stations varied by county. Along with the radios, PEMA created a talkgroup for every county that could be utilized by OpenSky users to contact the county. As we move to P25, the old OpenSky radios are being replaced by Motorola consolettes with the same talkgroup set-up. The radios I have looked at over the years at 911 centers generally have PennDOT's main talkgroup for the county's maintenance office, PEMA statewide, PEMA regional talkgroups for whatever region the county is located, then county talkgroups for the home county and neighboring counties. Some counties could patch their local systems onto the talkgroups when needed, some could not...it all depended on how the radios were set up locally. I would imagine it will be the same way with the new P25 radios. Replacements started in the NW and will work through PEMA Western Area, Central Area, then Eastern Area counties over the next year or so.
Edit: If I recall correctly, the radios also had regional taskforce talkgroups in them? I don't recall ever seeing these utilized. I do know caches of OpenSky portable exist with PEMA regional taskforce talkgroups programmed. I saw a bunch during the Papal visit, although I don't recall them actually getting used as everybody and their mother had their own radios there.