Cisco phone ringing even after call pickup

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.
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José MéndezCommented:
Please verify my understanding of the situation:
- Users in site A calls user in site B
- User in site B hears the phone ring (what phone models by the way)
- User in site B picks up the call, what does she hear or experience?
- User in site A hears ringback tone even after the user in site B answered?

Once you clarify those details, please configure and collect Callmanager service traces from all CM Nodes as shown below:

Attach them here and provide calling and called numbers involved in the failed call.
The few comment limits the exposure.
Adding to the suggestions Willy made is to determine whether it is an issue  with the phone on your end whereby the headset/handset going off hook is actually not activating the extension selection.
The other option is that the dialed extension is a part of a group(blast, hunt, etc.) that consists of multiple extensions....... And when is picked, there is a delay that some others ring an additional time.
Main part is to identify the conditions/caller/called to narrow down your debug capture.
Vontech615Author Commented:
I appreciate the suggestions. The issue ended up being a codec negotiation problem between the 2 clusters.  What happened was one side of the trunk was configured with a specific Device Pool.  That same device pool was not configured as a region relationship within the site's region settings.

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Vontech615Author Commented:
I solved this myself but wit the help of the suggestions.
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