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Random issues with DTMF for different IP Phone models


I need some help trying to figure out what needs to be done here.

We have CUCM8.5.1.13900-5 with a Cisco 2901 (acting as a CUBE router) with a SIP trunk terminated to SPRINT.

The voicemail solution is Exchange 2010 UM

Everything works, but DTMF results are different based on the phones used (through the SIP trunk or to the UM server).

- 7960 phones - DTMF works!

- 7940 phones - DTMF does NOT work

- 7941 phones - DTMF works for 2-3 calls, but stops working after that.  I need to unplug the phone to have it working again.

This is whether it's internal or external calls.

What can I do to get DTMF to work correctly for all phones?

Thank you!


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