Help with intercom / announcement dilemma

I have a client that has a retail operation that is normally staffed by just 1 person. There is additional office staff in the back of the building. They are looking for a way to have a button on the phone (yealink t28), that would alert the people in the back that help is needed in the retail shop (suddenly busy, suspicious person enters etc). Just press the button and it’s done, no hang up needed, or having (or even wanting) to say anything.

I’m trying to think of ways I could have a speed dial on the phone, that would trigger a simple announcement (“help needed up front”) to be played on the phones in the back as an intercom. My brain is failing me on this idea though.

I know a simple push button/remote bell would be an easy $20 solution, but the office is fairly spread out, and several people are in individual offices, so I’m not sure they would hear it.

Any ideas?