Session Initiation Protocol (SIP) Application Layer Gateway (ALG) is a feature enabled on most routers by default which can interfere with SIP and effect VoIP functionality. If you are using a Draytek router, you can check if SIP ALG is enabled and disable it by the doing the following:
1) Logon to the Draytek via it’s web interface (usually http://192.168.1.1), under ‘Management’ ensure that ‘Telnet’ is enabled.
2) Use Telnet via the Windows Command Prompt, or Putty to establish a command session with the router.
3) Issue the following commands:
sys sip_alg ?
Checks if SIP ALG is enabled or not, if it is:
sys sip_alg 0
Saves changes to the router and restarts the device.