Yes, if just a lit key will do it. Print the DESI tab to whatever you want it to say and then use a feature under the key that would light all the keysets programmed with the same button mapping. Use an RP key or something like that - anything that would light at all phones. You could make the RP the real deal and add a chime into it so when the next call comes in on any of their DIDs, all the phones would ring and the chime. The robber wouldn't know the difference. Maybe you could have all the calls redirected to a message that says 'we are being robbed, please call the police....?'
The only way I could think of making the button flashing would be to make it a DS button to a single line or phantom, but that wouldn't be great because the initiating phone would be ringing over their speaker and ... bang! I am sure there are liability issues for the bank in setting up such a system. If someone was harmed trying to 'signal' other people via the phone; unusual procedure. Just make sure you get a liability disclaimer from them.
