SIP ALG (Application Layer Gateway)
SIP ALG is an Application Layer Gateway which acts as an intermediary between your system and VoIP Much. SIP ALGs actively monitor and often modify SIP packets. The intended purpose of a SIP ALG is to assist SIP phones located behind NAT devices (routers/firewalls).
Frequently, poor implementations of SIP ALGs create issues including one-way audio, dropped calls, run-away calls, and fax failures.
You may look into your network equipment and identify if there is a SIP ALG currently active.
We recommend that you disable your router's SIP ALG, which may also be referred to as something along the lines of SIP Helper or VoIP Assistant.