Iphone 6 plus will not default to SMS when other user iphone is not on wifi My iphone 6 plus will not revert to SMS when another iphone user is off wifi.
No problems before when I had an iphone 4s full plan and hubby had iphone 3gs go phone (no data) the imessage app worked great.
Now the iphone 4s is a go phone (no data plan)upgraded to ios 8.1.1. The iphone 6 plus is also 8.1.1. full plan.  The send as SMS is on on both phones.  

Is the difference that imessages makes you sign in with an apple id which is an email address?  The phone number and email is checked on both phones for receive and send.  What a pain that my imessages never get to my hubby until he gets to wifi.  The 6+ does not sense or revert to SMS like the 4s always did.
If any capable device receives the iMessage, it's considered delivered. 

Log out of iMessage all other devices/computers &amp; then it should resend as SMS.