I have two phones in the same office and I need them to ring out with the same phone number

I have two phones in the same office and they are currently both ringing out with different phone numbers. For example, when you call a customer from one of the phone the caller ID number shows up as 123-456-7890, while the other phone shows as 012-345-6789 (examples obviously). I need them to both ring out with the same number. I have two other offices set up this way but for some reason it is not allowing me to set up these phones that way.