We have a Gendband C15 softswitch that had a bit turned on that did not allow RCF 2833 DTMF. When trying to access voicemail with both VVX 501 and a VVX 601 (firmware 5.5.3) the DTMF tones would not pass to the VM or conference system. This is now fixed by Genband allowing RCF 2833 in our softswitch, but we thought that the phones should have reverted back to use G711 once our softswitch said not capable to do RCF 2833. Is there something in the config of phones that we could have set to have phone revert to use "in band" DTMF tones? The phones did work fine with older firmware 5.4 1.
Have you taken a packet capture from the phone to see what dtmf tones are being sent?
As for the audio codec g711, you have the option of selecting this or forcing this from the web page of the phone under Settings > Audio codec priority.
Regards, Karthik Sivaram
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