Ph number greyed out.

I have iPhone 6. In settings under send and receive---- then send and receive iMessages from ---my cell ph number is greyed out and only email can be choosen. I only have Wifi.

I can send txts no problem to android users when out of wifi range but when I try and send a txt to friends who are iPhone users when out of wifi range it does not revert to a sms message even though it is turned on.

.All i get is "Cannot send message as iMessage needs to be enabled." Which means having to find wifi!!

Very frustrating.Does it have anything to do with my cell ph number being greyed out.

Have tried all the helpful hints to get ph number in but to know avail!

The phone actually states it will send as SMS when iMessage unavailable.