I recently got an iPad mini and already have an iPhone 5. I set up messages on the iPad, but now when some people send me messages, they sometimes go to my iPad instead of my phone.
You must be using your Apple ID email address as the contact for Messages. If you want to receive the messages on both devices, you need to add the phone number in Settings>Messages>Send & Receive at>You can be reached by iMessages at...see if your email address is checked in there. If it is - and if your iPhone number is in that setting - check it to use as the contact address.
If the number does not appear, you can add it, but you will probably have to starta ll over again - sign out on all devices - including the phone.
I had one person change their Messages to use only SMS so it wasn't eating away her data plan, but I've got some other friends whose messages go to the iPad also.
Using only SMS does eat at the data plan, using iMessages via WiFi does not. You can send SMS messages from iPhones and other cell phones but that works over the carrier's network so it does use the data plan. iMessages will work over WiFi and will not use the data plan at all - as long as the message is being sent over WiFi.
The problem is that the messages are showing up on the iPad, which is Wifi only and not always on me, instead of showing up on the iPhone, which is always with me. People are texting me and I'm not getting it until I get home and check on the iPad. Pretty inconvenient. I want them to show up on the iPhone ONLY, not on the iPad at all.
Assign a different address (gmail,com?) to your iPad.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
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iOS: About Messages
Set up iMessage
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To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
You can check the status of the FaceTime/iMessage servers at this link.