Correct detection of bitrate

From reading the speex manual it looks like the bitrate is part of the encoder output (4 bits in the first byte) ???
 
So here is a scenario I would like some feedback on:
 
Client A makes a narrowband Speex VOIP call to Client B.
 
In the SIP negotiation there is no information exchanged besides the fact that it is a narrowband speex call (speex/8000)
 
Client A operates at a bitrate of 11kbit/s (28 bytes per payload)
 
Client B operates at a bitrate of 8kbit/s (20 bytes per payload)
 
So client B receives a payload of 28 bytes.
 
Is the application on B's side supposed to extract the mode bits and reconfigure the speex decoder _before_ the payload is
given to the decoder? I don't see how it would work unless the decoder is reconfigured to operate at the same bitrate as
client A's encoder???
 
And how is B guaranteed that A will also reconfigure its decoder to operate at the same bitrate as B's encoder? Or should
the encoder on B's side _also_ be reconfigured to use the same bitrate as A is using once the mode has been detected? That 
looks like a cyclical problem to me...so what am I missing here???
 
Thank you.

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