Sms not working in iMessage

Question

This was all working fine linking with my iPhone 5s, then I suddenly noticed I was getting SMS on the phone but not on iMessage. When I try to send sms, it just reports them as 'Not Delivered, Could not be sent'.
Nothing has changed on the phone or the Mac (although this could go back as far as the last iOS update, cant be sure.
Logged phone out from Apple ID (message and facetime), logged facetime and imessage out on the mac. Logged both apps on the phone first, then facetime (works fine), then imessage.
Logins all report they have worked and all devices show up where they should, but still cant send or receive sms from Mac (works ok on phone)
Help!

Answer 1

Hi,
Both the iOS and the Mac OS X have apps called Messages.
They are different but compatible.
On enabling the app on the iPhone it can send iMessages as it registers the iPhone Number with the iMessages Servers. If you try to send to an Non Apple device or one that is not Registered then it will send SMS to the Phones involved (and tell you it can't to the others).
The Mac App registers an Apple ID with the iMessages servers.  However it can also join 4 other services.
In the latest software on both devices you can no link the iPhone and Mac for SMS.
It does need both to be linked using the Apple ID on the iPhone so that iMessages that are sent display on both devices. Then it needs both to be on the same network.  Generally this means the iPhone is WiFi whilst the Mac can be Ethernet or WiFi.
The iPhones can always send SMS when they have a carrier service.
The Process that links the two uses the iPhone to send the SMS.
Resetting can be a good place to start.
As the iPhone can send SMS and this is a choice after activating the app on the iPhone it suggests that in Send and Receive on then iPhone that the Number is showing active for being linked to the iMessages servers.
Removing the Apple ID should be followed by making the app break with the servers.
I use putting it in Airplane Mode for two minutes and then going back to the Settings.
This will tend to cause the iPhone Number to Verify again with the servers.
After that has finished Add back the apple ID.
This should trigger the pop ups on the Mac version which you should accept (it re-links the Apple ID and iPhone number even though they still appear in the settings).
After that reset the Text Forwarding on the iPhone which should trigger the Code (a new one) appearing on the Mac version.
The Code number needs entering on the iPhone.
8:40 pm      Wednesday; March 18, 2015
​  iMac 2.5Ghz i5 2011 (Mavericks 10.9)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 Couple of iPhones and an iPad