iPhone 6S no longer operates apartment buzzer.

My apartment building has a buzzer to allow people in the front door of the building. Like most apartment or building buzzers, it operates by allowing users to operate the door lock remotely by pressing the "9" key on a touchtone keypad when somebody dials up.

I've used the buzzer for the last 2 years with my previous phones (iPhone 5S and Samsung Galaxy Nexus); but, with my recent iPhone 6S purchase the buzzer is failing to operate 90% of the time I try to use it.

The buzzers listen for the 9 key's tone; but, it appears that the new phone or software does not play the tone for long enough when the button is pressed. So, while 9 appears on the screen after pressing the key, I can only get the buzzer to work by repeatedly mashing and holding the 9 button and hoping it plays the tone for longer. I've only been able to get this to work twice out of 20 or more tests.

Is anyone aware of a way to solve this problem?

It's quite important that I'm able to operate the door remotely as I receive deliveries on a nearly daily basis and this prevents me from allowing the delivery person from entering to drop of the package.