Tablets, unless you have a SIM and service through a carrier/service provider, don't have phone numbers. Actually it is the same with a phone. You can have a phone, but without it having service, it can't have a phone number. Well not a standard one anyway.
There are apps you can use to make calls from devices (and have a phone number), as long as you can access WiFi. However I can't say if that phone number would work. Depends on what they want it for. It seems rather odd that getting an app requires a phone number. No app needs that kind of info.