Selection of media (m=) at MG

For the ISDN UDI-TA capability, [ETSI TS 183 036] proposes to describe 
the media at MG using the next SDP description in the Local/Remote 
descriptors:

m=audio <puerto> RTP/AVP <Dynamic_PT> 0 [8]
a=rtpmap:<Dynamic_PT> CLEARMODE/8000


MG must use the codec:

* CLEARMODE: in the phase of the call of information transfer.

* Audio Code G.711-u/A: in the phase of the call of establishment/release.


The phase of the call is known by the MGC (using signalling Q.931, not 
visible to the MG). But, with that SDP description, how may MGC indicate 
to the MG which codec may use at each instant?

Regards,

Javi