How do I register a phone number with iMessage? I know it's an iPhone i've texted before

[iMore Question]

When I try to call the number a message comes up saying this phone number is not registered with iMessage; in a second line it actually lists the phone number. I've texted my sister before at that phone number and it is an iPhone. I've searched all over on the web trying to find the answer and studied all of the options in iMessage to no avail. please help.

 

[Bigeric23]

If you both follow these instructions it should work.

https://support.apple.com/en-us/HT201349