A transcoder will need to register to something- it uses SCCP and this is Master/Slave.
You should define start the telephony-service on the router and register the transcoder locally to the telephony-service on the router (you are not really going to setup the router to become a CME- this is just for the transcoder).
I think the bit of config you are missing is:
sccp local Vlanxxx
sccp ccm 126.96.36.199 priority 1
ip source-address 188.8.131.52
sdspfarm units 1
sdspfarm transcode sessions x
sdspfarm tag 1 mtp
In your dial-peers do not use the voice class codec command otherwise you will not be able to use the transcoder- hardcode the codec to g729r8/g711ulaw...
One more thing- unless its a new feature, transcoding is not supported SIP-SIP. So you may have to use H323 on one leg of the call. Thats how it is in earlier 12.4 releases.
The transcoder needs to register to something- it can't invoke itself. You always need to associate it to you SCCP application that you define on the router and register it to one of CME/SRST/CCM. The document also states this, so I am not sure what you are saying?
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