adding numbers into a register

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Python

I am trying to simply add a integer that the user types in, and put that into a register.
I am trying to check that what the user has typed in is in fact a integer and also 11 numbers long before adding it to a list (register).

register = []

def add_number(number):
    if isinstance(number, int) and len(number) == 11:
        register.append(number)
        
add_number(raw_input("add a number to the register"))

print register

I have googled this and tried many ways to do this using things like type to check if it is a integer or converting the input into a string first then checking if that string matches 1234567890 but am confused why this wont work. I have got a function to work similar to this one but only with names instead so i can't understand why this wont work.