IAX codec packetization ignored

When attempting to set codec packetization size e.g allow=alaw:40
it works perfectly in sip.conf, but doesn’t work within iax.conf, instead it just uses the default codec packet size - in this case 20mSec.

I have reduced iax.conf to the following for test purposes

[general]

allow=all
codecpriority=disabled

[iaxtest]

type=friend
trunk=no
disallow=all
allow=alaw:40
context=iaxtest

I am using asterisk-1.4.21.2

Any idea’s?