Allocation resources for a call

I have a callwith the following 3 phases:

A) Phase A: MG may carry media to a equipment A using the codec x.

B) Phase B: MG may carry media to a equipment B using the codec y.

C) Phase C: again, MG may carry media to the equipment A using the codec x.


In that case, what is the most appropriate way to model the call on the 
gateway to ensure that once the call starts the gateway will have the 
necessary resources for all phases?

(i) 2 terminations (in the same or different context?) using the 
property "StreamMode=Inactive":

* Termination 1 with:
     v=0
     c=IN IP4 IP_A
     m=audio port_A  RTP/AVP codec_x

* Termination 2 with:
     v=0
     c=IN IP4 IP_B
     m=audio port_B  RTP/AVP codec_y

In phase:
     (A, C) Termination 1 with StreamMode=SendRecv; Termination 2 with 
StreamMode=Inactive
     (B) Termination 1 with StreamMode=Inactive; Termination 2 with 
StreamMode=SendRecv

===

(ii) 1 termination with 2 session descriptions with ReserveGroup=true:

In Phase A:
LocalControl {
ReserveGroup = True},
Local/Remote {
     v=0
     c=IN IP4 IP_A
     m=audio port_A  RTP/AVP codec_x
     v=0
     c=IN IP4 IP_B
     m=audio port_B  RTP/AVP codec_y
}

In Phase B:
LocalControl {
ReserveGroup = True},
Local/Remote {
     v=0
     c=IN IP4 IP_B
     m=audio port_B  RTP/AVP codec_y
     v=0
     c=IN IP4 IP_A
     m=audio port_A  RTP/AVP codec_x
}

In Phase C:
Local/Remote {
     v=0
     c=IN IP4 IP_A
     m=audio port_A  RTP/AVP codec_x
}

===

(iii) 1 termination with 1 session descriptions with ReserveValue=true 
to allocate the resources:

Phase A)
LocalControl {
ReserveValue = True},
Local/Remote {
     v=0
     c=IN IP4 IP_A
     m=audio port_A  RTP/AVP codec_x  codec_y
}

Phase B)
LocalControl {
ReserveValue = True},
Local/Remote {
     v=0
     c=IN IP4 IP_B
     m=audio port_B  RTP/AVP codec_y  codec_x
}

Phase C)
Local/Remote {
     v=0
     c=IN IP4 IP_A
     m=audio port_A  RTP/AVP codec_x
}



Regards,

Javi