Conference Problem

I am using a Xeon server as an asterisk server (asterisk 1.2.4) and moreover the SIP phone is Grand Stream BOX with codec as Alaw
And calling through IAX trunks with codec as GSM

What problem are you experiencing?


i am facing problem of low voice at the time of conf. and i am using ztdummy.

I am using a Zaptel FXO card as a timing device. Is there any advantages to using ztdummy?