Update: I put in another router in front of the ER605 which has only my IP phones and the ER605 connected to it. The phones now work fine, and I didn't have to reconfigure the rest of our LAN. So...

@Virgo Thank you for your reply. Yes I did try to configure my phone's IP in the DMZ, but still no luck.

@smyles I checked and it says my IP address is not blacklisted. So it has to be a config setting on the router. I mean the issue literally started happening after the router was factory reset, so...

Here's my device's menu:

I don't see these options on my ER605v2. The menu is different to your screenshot and I don't have State Timeouts anywhere in the interface, I've check every tab.

@d0ugmac1 Tbh I have now tried 2 different providers. After our current provider wasn't able to help, I signed up on a trial with RingCentral and I'm getting the same result.