Unable to activate iMessage on my iPhone

imessage is an IP based P2P messaging service using Apple servers.

Imessage sending & receiving works with both your carrier phone number(GSM phone number) & apple id email.

imessage Activation

It has to be activated at two levels

a) device level: Go to settings —>apps—> messaging app — imessage -ON

b) mobile carrier routing - generally for first time if using GSM phone number to send/receive. Please note, this step is optional if you are using only apple id email to send and receive. However this is an important step for GSM phone numbers that happens in the background and your mobile carrier must enable your GSM number for lookup otherwise sending iMessages using phone number WILL NOT work.

Also note that some network providers charge (for e.g Airtel India) for activating iMessage first time and continue charging each time. Basically they charge you to enable your phone number lookup within their perimeter network allowing someone to lookup your number in apple’s imessage server registry when sending SMS, and if available it turns the send arrow to BLUE.

when sending , BLUE POINTING UPWARDS ARROW says to you that it is an imessage and it will be FREE. If the arrow is GREEN, it is not an imessage and may be chargeable.

iMessage send & receive settings (after turning ON imessage)

Setting—>apps—>messages—>Send & Receive

use phone number (international format) AND/OR

use Apple id email

Important things to note

sending & receiving imessage's using Apple id email is always IP based, as there is no mobile network’s DNS server lookup dependency.

However if you are using GSM number,

Activation must be done by the carrier prior to sending. This is because the phone’s app will look for the Apple imessage server IP (DNS lookup) via the mobile carriers DNS servers.

After activation, it should normally stay ON as long as your mobile number and account is active

However, some operators (Airtel India again) is known to deny DNS lookup sessions especially when the phone is abroad. So your apple phone thinks it is a normal SMS and will unknowingly result in SMS carrier charges for sending messages as normal SMS.