Our company recently converted to a 6 digit extension from 4. We merged with a company that had 6 digit extensions so this was an attempt to get everyone on the same page. The other company still has there own pub with phones at their sites registered to it. We haven't brought them over to our new CUCM. I have reports from a few users at one of our sites that calls from the other company's sites will ring, and keep ringing even after call pickup.
Additionally, there is a SIP trunk between the 2 clusters.