Sometimes ALG (Application Layer Gateway) can cause problems with
ALG is often enabled on newer routers and can affect SIP (Session Initiation Protocol).
If you have a Draytek router you can disable ALG but have to do this via Telnet.
Once conencted via Telnet check the ALG status by typing in:-
sys sip_alg ?
If it is enabled you can disable it and restart the router by issuing the following commands:-
sys sip_alg 0 sys commit sys reboot