Recently had a few issues setting up my Billion BiPAC 7401VGPR3 to make and receive VOIP phone calls via the good people at Exetel. Solved all the issues by following these instructions:
Make sure SIP is enabled
Registrar Address(or Hostname): sip1.exetel.com.au
Registrar Port: 5060
Expire(seconds): 3600
User Domain/Realm: sip1.exetel.com.au
Outbound Proxy Address: sip1.exetel.com.au
Outbound Proxy Port: 5060
Phone Number: Your voip phone number 07**** ****
User Name: Your voip phone number 07**** ****
Password: Your voip password – You can find this in your Exetel Account: Manage Services – VoIP > Change Voip Service
Display Name: Exetel
Save these settings and you should be good to go! Worked for me.

Do you have some projects that i can call to USA for free?