It could almost be anything. What is different in your office from the other offices or areas. The magnetic white board I don't really think would be the problem. The wireless mouse won't affect it. I have had 2 wireless mice and keyboards, all bluetooth, in my computer room with my old Verizon phone and my ATT IPHONE 4 and have not any problems. I have had 2 wireless routers, dual band, and still no problems.
I have seen insulated/reflective windows, for keep out the heat, cause problems but I would think that these windows would be in every office in the building.
Are the other offices on different side of the building? The reason I ask it may be an equipment problem at the cell site that the phones in your office access but in the other offices the phones see or have a clearer view of different sites.
I don't know if the cable TV people in your area are on fiber or still on "copper". But the high end of their channels go right through the 800 and 900 MHz cell phone bands (
North American cable television frequencies - Wikipedia, the free encyclopedia). In my work it is not un-common for cable TV to be the culprit for radio interference.
Even worse, I worked a problem in the central New Jersey shore area. the problem turned out to be a bad TV antenna amplifier on the roof of a condo. My problem was around 118 MHz but these things are made to work up to almost 900 MHz. What made this problem hard to find was it was there 1 day and then "gone" for the next 4 months. Then it showed up again.
Another thing to try is turning off all the electronics in your office. See if you get a good signal and if you do start turning each piece of equipment on then waiting for about a minute or two and see if you loose the signal. You can turn everything on and leave it on when you turn the next piece on. If you loose the signal at a point it could be 1 of 2 things, either that individual piece of equipment is putting out interference or the cumulative noise from all of the equipment is causing the problem.
Good luck.