Zyxel equipment is often deployed by DSL providers such as Qwest and Century Link. If you are using a VOIP telephone and experiencing call control or audio issues you may need to disable the SIP ALG feature.
1. Telnet into the router from your LAN.
2. Select menu 24.
3. Select menu 8.
4. To display current SIP ALG status type in “ip nat service sip active”.
5. If it returns a value of “1” the SIP is enabled.
6. To turn off the SIP ALG type in “ip nat service sip active 0”.
7. Reboot your VOIP phones.
Networking and VOIP notes 
SIP has problems traversing certain NAT devices. You may experience One Way Audio