Log on to the router via Telnet using an SSH client such as Putty: http://www.putty.org/
Check if SIP ALG is Enabled or Disabled:
Enter the following command to see if SIP ALG is Enabled or Disabled: sys sip_alg ?
When you see 0 that means SIP ALG is Disabled and when you see 1 that means SIP ALG is Enabled.
Disable SIP ALG
Enter the following commands to Disable SIP ALG:
sys sip_alg 0
Now that changes are saved and the router will restart.