My ipad won't add a phone number

To send/receive cell calls and SMS messages on an iPad, you need to pair/tether an iPad to an iPhone to be able to do this using cell call and SMS message forwarding.

Tap and read these links below.

Make and receive phone calls on iPad - Apple Support

Set up Messages on iPad - Apple Support

What is the difference between iMessage and SMS/MMS? - Apple Support

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support

Good Luck!

Do you have an iPhone - or just an iPad?

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.

However, when paired with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s continuity features.

More information about Continuity and the relevant Cellular services can be found here:

Continuity

Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support

SMS/MMS Messaging

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

WiFi Calling

Make a call with Wi-Fi Calling – Apple Support

If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID - to send SMS/MMS messages (green bubbles) from your iPad you must explicitly enable Text Message Forwarding for your iPad on your iPhone...

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.

Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON

iPads are NOT iPhones/smartphones.

What are you trying to accompish, exactly?