Most late-model Netgear routers have a SIP ALG built in. This setting is enabled by default. When using a VOIP provider who utilizes an SBC (voice firewall) you may need to disable this setting.
Here is how to disable the SIP ALG :
1. Log into the router.
2. On the left side of the screen you will see “WAN Setup” under the Advanced section. Click on “WAN Setup”
3. Click on the box next to “Disable SIP ALG”.
* If you have active calls, they WILL be disconnected or audio will be lost when you click on “apply”
4. Hit Apply.
5. Reboot your phone.