During modern SIP outgoing and incoming VoIP calls, ALG modifying data packets is problematic, especially considering how poorly implemented the service is on most commercial routers. The ALG service intends to provide clearer connections, but since it modifies data packets erratically, the opposite is often true. Each time the system confirms and acknowledges a connection attempt, the ALG modifies the SIP packets being sent. This is because the changeover from private IP addresses and ports to public is done by scripting. Scripting is very risky - sometimes during the translation process, important parts of the message are lost. This will affect VoIP calls in different ways:

Failed Registrations - With multiple acknowledgments required during a call, if any fail, the call will fail to connect. This is called a failed registration, which is often a direct result of a SIP ALG working in the background.

One-Way Audio - Can you hear the other party, but they can't hear you? These one-way audio SIP calls are typically the result of either poor firewall settings or the ALG modifying the packets so that audio is lost on one end of the call.

Dips in Call Quality - When packets are lost during any internet-based call, you'll start to hear static, lapses in sound transmission, or echoing. Modifying the call degrades data during transmission or receipt.

Lost Connections - You can easily lose the call altogether with this router service. When data is lost and is unrecoverable, it's easy to be disconnected.

With a SIP call, there are more chances to incorrectly modify data packets in unexpected ways, which adversely affects your calls. Fortunately, disabling the router service is usually done easily in the router web interface. If disabling the SIP ALG feature isn't available on your device, then you should consider purchasing a new router.