Is there any change in the price depending on when I get the phone number?

I understand that at ZoomPhone, I have to pay when I purchase a paid call slot,
Will I have to pay again when I get the phone number?

I am wondering if I can get a phone number at any point in time and not lose money.
I am wondering if there is any difference between "I will not assign a ZoomPhone to an employee until 2 months from now, but I only need to get a phone number now" or "I will get a phone number right before I assign a ZoomPhone to an employee."
Are the charges the same for both?

Thank you in advance.