PUB CUCM not working

Hello guys,

I have 2 buildings, one building has a PUB and a SUB CUCM, while the other side has only 1 CUCM.

the buildings are connected together via a VPN.

when the building having 1 CUCM, points to the PUB CUCM in the other building, the calls don't work.

when it points to the SUB CUCM, the calls work fine.

I restarted the PUB CUCM in the building containing the 2 CUCMs, the calls work for some hours and then stops.

so every time the building with CUCM points to the PUb CUCM in the other building, the calls stops.

i have realised also that on the voice gateway in the site that has the 2 CUCMs, the dial peers connected to the PUB are saying "BusyOut" status. when I changed the dial peers to point to the SUB CUCM, the status changes to UP and the calls from the PSTN works fine.

any help PLEASE.