Back in iOS 8, users couldn't text iPhones. They could only text another iPad. If they wanted to text an iPhone user that had a phone number to text, they would have to download a third-party texting app. Now that iOS 9-9.1 has rolled in, users can now do this. They no longer have to get an Apple ID from an iPhone user to text. They can now get an iPhone users phone number.
You can still text another iPhone by getting the iPhone user's Apple ID, but the question is how do you use the regular built-in Messages app to text an iPhone when the iPhone user only gave you their phone number associated with their Apple ID?
The answer is iOS 9.1. It allows this feature.
I. Get someone's iPhone phone number. Head over to your iPad and tap the Messages application.
II. Tap the Compose button. In the To field, put in the iPhone user's phone number.
NOTE: I recommend adding the iPhone user as a contact first.
III. In the message box, put in your message and tap Send. The message has been delivered.
Note that if you are still on iOS 8, tapping Send will not deliver the message and Not Delivered will be shown instead of Delivered.