Hey,
You ain't missing something, that's the whole purpose of the SIP inspection ALG, make sure that value inside the SIP payload get's translated properly.
If that is not taking place then you got to be hitting a bug.
Regards,
Jcarvaja
CCIE 42930, 2xCCNP, JNCIS-SEC
For inmediate support http://iNetworks.cr
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC