Where is the phone number listed on the ipad exactly? Under settings and then where?
Are you logged into the web? similar to this https://support.apple.com/en-us/HT204265
on ipad go to settings --> icloud --> sign out/ back in here
test it now, it may take a few minutes to update on ipad
where else is this Apple ID signed into?
sign out of it on all devices as it's probably syncing from some other device
so the same apple ID is logged into wife's iphone and the ipad?
Can we create a separate ID for the ipad?
Otherwise log out of the account on the 2 iphones and stay logged out to see if the ipad will update the settings.
Checking in to see if you still need help with this issue, if so please reply at your convenience.