Why can't I send SMS or MMS messages?

[iMore Question]

When I tap iMessages setting send & receive. I do not get the SMS or MMS line. When I try to send to a known phone number through iMessage, the send function won't work & the number goes Red. I also tried same number through Safari & message "failed to deliver". My iPod & iPad are both the same & won't send from either one? Why?

 

[kch50428]

iPhone - Phone, Messages & FaceTime - Apple Support
If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support

 

[Massie]

Are you trying to send SMS to a non-iMessage device on your iPod or iPad? If so, that would be why it isn't working...SMS to a regular phone number uses cell service. If you're using non-cellular devices you will only be able to send to other iMessage addresses, which will turn blue when you enter them in the address field.