Using firebase database value as argument of function

Ionic Framework ionic-v3
1

Hi,
I have one firebase database, name branch, one of the key(phone) has value(607123456),
Inside html page, I can use {{ (branch | async)?.phone }} to get the value.
But now I have a function “callUs(stringNumber)”, how do I use {{ (branch | async)?.phone }} as the argument of this callUs function? So that I can pass different phone number to this function.

“callUs({{ (branch | async)?.phone }})” gets a template syntax error.

Somebody help. Thank you