Since moving to our current location, we have discovered that it is vital to be notified when the front or back doors are opened to provide the best service to our members. The normal solution of a bell or similar "low tech" solution isn't sufficient in a building that might find us on the opposite end of the building from where our members or the general public enter.
We started with an off-the-shelf push button doorbell solution. I found this Phillips DES2280W/27 wireless doorbell kit with a plug-in base station and wireless buttons. This particular model can be paired with up to four buttons in total. I decided that if it could have additional buttons paired to it, the possibility of being able to pair a button to multiple base stations was pretty high.
After purchasing the kit the first thing to do was determine if it was possible to tie the pushbutton into a magnetic reed switch or something similar to ensure it operated when the door was opened.
After removing the cover from the wireless doorbell button, I found a simple PCB with a surface-mount tactile switch that causes the base station to ring. A simple jumper wire proved that I would be able to solder a new switch in parallel to the existing tactile switch and get the doorbell to operate in the way that I wanted. So, I opened up my CAD and made an enclosure to hold both the reed switch and the button PCB. I purchased my reed switches locally, but if you cannot find them, you can always order something similar to this online.
Once I had my enclosures printed, my wires attached, and my reed switches secured to the enclosure, I could do a test fit with my magnets and ensure the doorbell operated as intended. I quickly discovered that the doorbell worked as intended but I couldn't hear it from the other side of the building. After purchasing a second doorbell base station, I could pair the buttons to both base stations, and when I opened the door, it rang the bell on both base stations. I repeated the same procedure for our back door and now I have a bell that rings if either door is opened to let me know that someone has entered the building.
I hope this inspires you to go out and create a solution to a problem.