So I opened a TAC case and the tech said to go into Call Manager, Device, Gateway, then locate your MGCP Gateway that is having the problem.. click on the T1 PRI endpoint and change the clock setting there to External Primary all of mine were set to ...

I'm getting the exact same problem on our ISR 4331. Cisco IOS XE Software, Version 16.12.03. causing Slip Secs on the PRI/T1 and errors. clock source line primary is missing from controller t1 after router reloads  Nominated InterfacesInterface SigTy...