Note that as this process may temporarily interrupt your internet connection and phone service, we recommend doing this in an off-peak period or outside of business hours.
Disabling SIP ALG
From the router's menu, select Network > ALG Settings. On some firmware versions, this has moved to Advanced > NAT Forwarding > ALG
Under Application Layer Gateway (ALG), find SIP ALG and check Disable. Click save and, if prompted, restart your router.