Problem with G.729 Passthrough and PSTN dialing

Hi,
After reading some I changed my sip_custom.conf and added:
disallow=all
allow=g729
allow=ulaw
allow=alaw
After that I was able to make 729 calls between two phones that supported G.729. However, I was no longer able to make outgoing calls via my PRI !!
Can any one shed any light on this ?
Also, why the line disallow=all ? why now simply allow all and that is it ?
PS I do not have any G.729 license. but as G.729 calls worked I do not think that is relevant.
Thanks