Most banks do have a silent alarm system that the teller can press to summon the police.
However it's usually not safe to try and press the button until after the bank robber is gone which kind of defeats the purpose of the silent alarm system.
Usually the tellers are told to give the bank robber the money and then press the button afterwards so that the bank robber leaves the bank and hopefully does no harm.
When my mom worked in a bank she was always told to go alone with what the bank robber demanded and just hand over the money.
The money that she handed the bank robber also included a die packet that was set to go off after the bank robber got out of the bank.
If the bank robber stuck their hand in the money then the die would get all over their hands and then there would be a certain soap to get the dye off of their hands.
Then the teller would press the silent alarm to alert the police to the bank robbery and the police would then arrive and then go out searching for the bank robber.
It's best to let the bank robber get out of the bank before the police show up because then it could lead to a hostage situation and the bank robber could hold the teller at hostage when the police show up.
So they usually give the bank robber the money and then alert the police to avoid a potentially deadly hostage situation.