How do I let people in my building when I'm using my phone for other call

[Android Central Question]

When I am supposed to receive a parcel on a certain day and I happened to be on the phone doing something else when my buzzer rings how can I let people in because I have no access to the keypad to press number 9 at the time when I am talking to someone else

 

[hallux]

How is this alert coming in? Does this come in as a phone call? Would you be able to swap phone calls like on call waiting, act on it then go back to your other call?

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