Trouble in making conference calls..!

I m using asterisk 1.2.13 on CentOS. The problem is that i am not able to make any conference call through asterisk.
The problem is when i just use codec g729 and in the field disallow - all, it doesnt let me connect to a third call thru my cisco IP phone 7940 series.
But if i add u-law or a-law along with g729 codec it does allow me to make a conference call.
in third case if i just mention g729 codec and under disallow - “blank” then in this case it allows me to do a conference call as well. when i have a look at the asterisk by show channels and show peers, it says it uses u-law for the 3rd call.
can anyone please guide me on just using g729 codec and not u-law or a-law, for making a conference call, and whts the problem that is causing this.
please advise,

do you have the g729 codec module installed and loaded ? how many licences have you purchased ?

show us the sip.conf entries (or whatever technology you’re using) for the users concerned.

hi baconbuttie,

i have installed and loaded g729 codec and as for the license i am in australia so its free in here. i think we need to pay for the license if in US and Europe.
well as for the sip.conf contents then its is hereby as follows:-

; Note: If your SIP devices are behind a NAT and your Asterisk ; server isn’t, try adding “nat=1” to each peer definition to ; solve translation problems.


port = 5060 ; Port to bind to (SIP is 5060)
bindaddr = ; Address to bind to (all addresses on machine)
; If you need to answer unauthenticated calls, you should change this ; next line to ‘from-trunk’, rather than ‘from-sip-external’.
; You’ll know this is happening if when you call in you get a message ; saying "The number you have dialed is not in service. Please check the ; number and try again."
context = from-sip-external ; Send unknown SIP callers to this context callerid = Unknown

; #, in this configuration file, is NOT A COMMENT. This is exactly ; how it should be.
#include sip_nat.conf
#include sip_custom.conf
#include sip_additional.conf

register =>