@Pidge25 

Hello, the SIP ALG is enabled by default, there is no toggle to turn it on/off yet. 

While we can do some troubleshooting first and try to fix it:

1. How do you connect the VOIP phone to the internet? By wired cable or wireless?

2. With VOIP phone connected to the main Deco by wired cable, does it work well?

3. Try to change the operation mode from router mode to AP mode to give it a go. https://www.tp-link.com/support/faq/2400/

4. Besides, you can contact the tech support of the VOIP phone to get some help, like what does the error message mean, and what we need  to do to make it work. 

5. Or you can change the Skyq hub to bridge mode and use the Deco P9 as the main router to give it a go. Like point 3, there won't have double NAT issue. 

Good day.