UC500 Series Transcoding

When a device receives a call using a CODEC it doesn't support it will try to invoke a transcoding session to convert the RTP audio stream into a supported format, otherwise the call will fail. You'll encounter this problem on a UC500 when using a SIP trunk that delivers audio in G729 format as CUE only supports G711, thus callers are unable to leave voicemail messages.

The UC500 series when configured by CCA will already have a dspfarm profile for conferencing, if there's any unused DSP resources then transcoding can also be enabled. Amend the configuration below to match your IP address and the number of sessions your DSPs can handle:

voice-card 0
 dspfarm
 dsp services dspfarm
!
sccp local Loopback0
sccp ccm x.x.x.x identifier 1 version 4.0
sccp
!
sccp ccm group 1
associate profile 2 register xcodeprof
!
dspfarm profile 2 transcode
 codec g711ulaw
 codec g711alaw
 codec g729ar8
 codec g729abr8
 codec g729r8
 maximum sessions x
 associate application SCCP
!
telephony-service
 sdspfarm units 2
 sdspfarm transcode sessions x
 sdspfarm tag 2 xcodeprof
!

sdspfarm units should match the number of dspfarm profiles, in this case 2 - conferencing and transcoding. You can then confirm the DSP resources have registered with show dsp profile:

Dspfarm Profile Configuration

Profile ID = 2, Service = TRANSCODING, Resource ID = 1
Profile Description :
Profile Service Mode : Non Secure
Profile Admin State : UP
Profile Operation State : ACTIVE
Application : SCCP Status : ASSOCIATED
Resource Provider : FLEX_DSPRM Status : UP
Number of Resource Configured : 4
Number of Resource Available : 4
Codec Configuration
Codec : g711ulaw, Maximum Packetization Period : 30
Codec : g711alaw, Maximum Packetization Period : 30
Codec : g729ar8, Maximum Packetization Period : 60
Codec : g729abr8, Maximum Packetization Period : 60
Codec : g729r8, Maximum Packetization Period : 60

IMPORTANT!  CUE appears to have a bug whereby if the dial-peers directing calls to it use a voice class codec to determine the CODEC to use, CUE will try to use the 1st CODEC even if CUE doesn't support it (e.g. G729). To avoid breaking calls to voicemail, use codec g711 on the dial-peers to CUE instead. This issue also occurs if the inbound dial-peers from the SIP trunk use a voice class codec, the CUE module won't invoke a transcoding session, so again manually specify the codec.