How does the phone determine whether a text message is sent via iMessage or the user's regular cellphone service?

If Wi-Fi is available the message will be sent via iMessage to Apples servers. If no Wi-Fi is available the message will be sent via SMS texting.

Caristonia wrote:

I say this because I had texted a friend one time and it used the blue bubble, saying the text had been sent via iMessage. But I know for a fact that this friend does not have an iPhone and was not using his iPod to text.

May be he had iMessage active on the Mac ?

My whife uses iPhone and sometimes a Nokia with same SIM. If she has iPhone the blu bubble goes out in seconds and immediatly I see the note "delivered".

If she has changed to Nokia it starts blue but I do not get the delivered until the bubble changes to green 2-4 minutes later. I have activate the preference Send as SMS.

This seems doomed to failure. I can have imessage on my mac but turned off on my iphone. How does someone sending me a message know which method to use? If they use imessage my iphone won't receive the message because imessage is turned off on the iphone. If they send a regular text my mac won't receive the message since it only knows about imessage.

First, thanks for responding to a thread that is over a year old.

Second, a person sending you a message shouldn't have to decide which to use - it's automatic. If iMessage is available by any of your devices, the sender will use it automatically. If not, and they have the option to send as SMS when iMessage is unavailable, it will go out as a text ONLY to your phone.

If this is a major concern to you, do what I advise others to do. Log out of iMessage on all devices EXCEPT your phone, and you will always get every message.

Thanks for the reply. I see how your solution works. Is there a way to verify I've logged out of all imessage devices except my phone or do I have to find every device I've logged in to?