Apple - How do I send text messages to non-iPhone owners using the iMessage app on a Mac or the Messages app on an iPad?

On your iPhone, I'd try going into Settings > Messages > Text Message Forwarding and then uncheck the switches by whatever is not receiving SMS messages and then check and then re-authenticate them individually, it should only take a couple of minutes.

I had a similar problem when I changed SIM cards (though I also changed numbers).

To fix the problem I suggest:

• Signing out of iCloud on your devices and then (starting with your phone) signing back into all of them. This seemed to update my iMessage associated number (which doesn't seem to apply to your situation since you didn't change numbers - so perhaps you can skip this)

• Ensure that on all your devices iCloud accounts that your email AND (correct) phone number are listed. If your email isn't listed then add it (if your number isn't listed then repeat the above step)

• On your iPhone 6 go into settings -> messages -> text message forwarding and enable it on all of your devices (that should all be connected to the internet).

• Enter the relevant codes that pop up on your devices

Note: I think you'd really benefit from updating your iPad Air to iOS 8.whatever instead of trying to get it to work with iOS6

I'm also not sure if you're going to run into problems trying to get it to work on 2 phone devices. I'd try and get it working on all the other devices first and only then try and add your iPhone 5 into your ecosystem.

Helpful sites:

http://www.macrumors.com/2014/11/06/text-messages-phone-calls-on-your-mac/

Hopefully you get it working!

I had the same issues after I restored my iPhone 6. Going into Settings > Messages > Text message forwarding did the trick.

The only hangup was that the code took about 30 seconds to appear on my Mac's screen. And if you're impatient like I was you cancelled and tried it again some 5 times. So I'd suggest waiting at least 2 minutes before cancelling.

It works, afterall.