VOIP query

Hello all!!

I am working for a carrier company who uses ip network consisting of Cisco Routers to transport voice calls.The company deals mainly with pre-paid calling cards.The customer buys the card and dials a toll free no. to make a voice call or makes a call thro a dialer program(Installed on the pc) which sends the calls thro the ip network.When i make a call from a dialer on a pc which has a dialup connection,then absolutely there is no problem. But when I make a call from a pc which is on the Home LAN then only the destination party is able to hear my voice.I am not able to hear their voice.We r not using any firewalls as of now in the home but may go for it in the future.Nat has been configured on our home router and we hv a DSL connection to the ISP.I am not able to figure out the problem.can someone help me in identifying the problem

Thanx in advance..

Cheers
ss