Musical toilets.

Water can only rise in the pan if there's water higher than it in the s*it stack, or, in rare cases, there's a vaccuum, in which case it'll need a air inlet valve

They won't be connected, but if you're running a tap and the crapper's filling up, this is an indication that the stack's blocked and you are effectively raising the level of the blockage

It's probably air. If they want it fixing permanently, the boxing in will need breaking open and an air inlet valve fitting, unless you can get one outside. The stack can't already be open at the top or this would never happen

I assume there has been no problems in the past? If not, sounds like a partial blockage somewhere If you have drain rods and the flat rubber circular thingy that fits on the end.

Block off shower and wash basin drains, Use the plug and get someone to hold down or use a rag with someone stood on it.

Get a single drain rod with the flat thing on the end to use as a plunger, flush the WC and immediately work the plunger back and forth in the WC pan to scoosh the water as violently as possible. May clear it.

Is there a rodding eye outside? I may be necessary to dismantle the soil stack outside or put rods down the vent/rodding eye if there is one

Also lift manhole cover(s) outside & see if all seems normal when toilet is flushed.