How do I set up a separate texting number on iPad using an old US/Canada mobile telephone number?

If your iPad's were originally purchased with an AT&T Data Plan, the Cellular number associated with the plan is only used to identify the account for billing purposes. This number cannot be used for Cellular SMS/MMS Messaging, or for making Voice calls.

In more detail...

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity alone can only access IP-data services. An iPad is not, and can never be, a standalone substitute for an iPhone.

However, when coupled 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. Additional information about Continuity and the relevant Cellular services can be found here:

Continuity

Work across all of your devices seamlessly – Apple Support

SMS/MMS Messaging

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

WiFi Calling

Make a call with Wi-Fi Calling – Apple Support

Making and receiving calls on iPad

Make and receive phone calls on iPad - Apple Support

When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.

Probably by acquiring a new or porting an existing number to Google Voice or similar service.

Or by moving to a service such as Twillo or alternative.

Or by using an eSIM slot in an existing iPhone, if an iPhone is available here.

Apple hasn't put their C1 modem or another voice- and SMS-capable cellular modem into an iPad. iPad cellular communications are data-only. Not voice, and not SMS/RCS. (Qualcomm controls a fair amount of the cellular modem component business, and C1 is the first Apple cellular modem.)

TL;DR: You can’t get what you want. Not directly.

Any cellular circuitry in an iPad connects only to cellular data networks, not voice networks (which are also where SMS and MMS messaging take place, at least in the US).

Presumably you wish to use your iPad to send/receive SMS Text messages using your iPhone's AT&T Cellular service.

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

Mr.Schultz wrote:

Thanks all. Explained well. Too bad though. It would make sense if someday, Apple included telephone voice & messaging in an iPad. I guess it would kill sales but…?

It would require, as I understand it, a different chip/