With the information provided here, if the only change is with replacing the Framce relay, I dont see a reason why MTP would be required. Maybe what you are noticing is not the issue. When you enable the check box for MTP required, the call routing through your MTP server and not directly between the endpoint. With your VPN configuration, there would be a path/routing enabled betweent he endpoints and CUCM (I'm assuming it is using Software MTP on CUCM) ant the same is not allowed between the enspoints, I'd recommend doing the following checks first:
1. From the default gateway of the IP phone, do an extended ping (Using IP Phone default gateway as source) to the VG which has analog phone registered and vise-versa.
2) If the above works, then check if there is a firewall in between which does is blocked UDP ports 16000-32000.
Is the gateway on the remote site registered to CUCM? Yes
If it is, what protocol is it using to register? H323
Where exactly are you enabling the MTP required option? from the CUCM GUI > Gateways >Configuration of the VG there is a checkbox for Media Termination Required.
Before enabling that we found out that IP phone only can dial the analog phone, but when the analog phone will dial they only hear "Your call can't completed as dialed". I think the DTMF is not incomplete/no signal passing thru the CUCM.
After checked the MTP required, the issue was resolved. That's why we need to know why the Frame Relay no issue only on IP VPN connection.
"Your call can't be completed as dialed" means that the inbound Calling Search Space that gateway is using doesn't have access to the partition where the called number is. That is a generic CUCM message that has that function.
If this was a codec issue you would just get a fast busy tone. I guess someone has added the correct Partition the the CSS, you can check it disabling the MTP required option, if it still works then you have solved it.
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