[feat] Show phone ringing status on poster screens

[rinkp]

Mon-Fri between 9.00h and 12.30h and 13.30h till 17.00h, there usually is someone in the board room to answer the phone. Outside these hours, it happens often that the phone rings and that people cannot hear it due to loud chatter. Showing the phone ringing status on these screens will help with answering the phone.
I think it is fine to do this 24/7 and not limit the time, since we are not phoned that often.
I do think we should not display phone numbers in the room, rather "The phone is ringing". Then, we can display names and photos like we used to do for a selection of people.