Before buying new equipment you can try making some adjustments.

First, get the wireless access point (the ISP router) up high. Put it on a high shelf.

If the router has external antennas, make sure all of them are pointing straight up and parallel to each other. If the antennas are internal, leave the device in the orientation for which it was designed - if it’s made to stand on end like a book don’t lay it down.

If you have access to the configuration page see if you can set the wireless channels to auto, and the channel width to 40 MHz for the 5 GHz band and 20 MHz for the 2.4 GHz band. You can even try 20 MHz on both. Those smaller channel widths will give lower bandwidth but improve coverage due to hitting less interference. (Since you said a flat, I’m assuming lots of other residents with their own devices so we want to avoid interference.)

If auto channel isn’t an option, get an app like Wifi Analyzer and see what channels have the fewest other networks on them and set the channels to one of those.

Do you have 5g home service available? One “landline” ISP and one wireless ISP is a good way to get backup.

The IT way is to install a PBX system to manage the phones and transfer calls, but you need multiple phone numbers to make that worth it.

The SOHO/home way is just a punch down block that will electrically connect all the phone jacks together and have one line connecting to the POTS RJ11 port on your VoIP router.

The signal level is 0 in the radio itself. As soon as you go out a cable to an antenna you have loss. That’s why the value is always negative.

I’ve seen RSSI values as good as -26 using a good LTE router with a high end outdoor antenna very close to a tower.

If compliant with federal law, the behavior would be the same as in any other situation with DFS.

When starting to use any spectrum in the radar band, the AP would listen for 60 seconds before transmitting any signal at all. Then if a radar signal is detected it would change to a different channel. If that channel used radar band spectrum it would start another 60 second listening period.