In this case the SDP media attribute received in the incoming INVITE that CM does not like and rejects:
“Media Attribute (a): ptime:5”
ptime gives the length of time in milliseconds represented by the media in a packet. This attribute indicates the packetization period that the author of the session description expects when receiving media.
On the relevant “ip-codec-set” form on CM one can set the “Frames Per Pkt” which effectively changes the packet size: 1 frame per Pkt resulting in 10ms, 2 in 20ms 6 in 60 ms etc.
However, in CM, depending on the codec, (for eg G.711A/G.711M the valid entries: 1-6, 2 being the default which results 20ms packet size) valid entries vary between 1 and 12.
Thus when CM gets a "ptime:5" media attribute which means the author of the SDP wants to get the audio in 5ms packet length, unfortunately there is no such setting in CM and this results in CM dropping the call with "488 Not Acceptable Here (No Matching Codec or Encryption Algo)" response.
