imessage not activating

Hi. i have just bought an iphone SE for my daughter. she has already got an icloud account and used this to use imessage on her ipad. i have set up the new iphone, however imessage will not use the new phone number to send imessages, it will only use the previous email. on settings/ imessage it says activating.

the apple id on the phone and imessage sign in are the same. ( i think).

any ideas on how to get imessage to activate the phone number?

the phone can make calls and send sms with no problem.

Thanks