Yeah, I wondered the same thing when I was using FOP - I think it is a WebRTC phone specifically tweaked to work with FreePBX and the extensions in their default configs - @lgaetz could probably give us the exact scoop - see my adventures here: FOP2 WebRTC with Asterisk 18.3.0 and FreePBX15 working settings and a question - FreePBX / Tips and Tricks - FreePBX Community Forums
The way around this is to set up an extension specifically for the WebRTC phone, leave the physical phone on it’s normal extension, and put Follow-Me on both extensions referencing the other - so if anybody calls either extension, they both ring no matter what - not an elegant solution, but a workable one - You can also make the secondary extension look like it’s the primary extension through the advanced settings - then you only need FM/FM on the Primary to have it ring both.