I was implementing a SIP trunk from Verizon for a customer and I was skimming through the document specific for Verzion and CUBE on ISR 4K series and I realized on the CUBE it says early-offer forced but the trunk from CUCM to CUBE has the SIP profile configured with no Early offer which causes the solution to break and might be hard for some folks to Troubleshoot.
I haven't checked other documents on this page but they also might have this issue.
Cisco Interoperatbility portal
Document with Error
Hope it helps.
