This article explains how to do this using your mobile carrier or an app.
Wireless Carrier
Some mobile carriers allow you to use your number with multiple devices.
Two similar devices includeDIGITS from T-MobileandVerizon One Talk.
The same feature might already be enabled on your iPhone or iPad.
These types of apps don’t let you forward your “main” phone number to other devices.
Why Would You Ring Two Phones With the Same Number?
Maybe you want your home phone, office phone, and mobile phone to ring at the same time.
This makes you less likely to miss important calls.
The setup also lets you choose where to talk based on the nature of the call.
The huge investment is a deterrent which explains why it’s a fairly rare setup.
With one number, you might configure a series of devices to ring whenever there’s an incoming call.
Unless they’re intentionally added or merged, that is.