iMessage taking long to verify phone number

I received a new phone ,I set it up and every thing iMessage was working ,I took a bit long to restart my old one . I updated my phone and then my iMessage stopped working . I researched and went through all of the necessary procedures but nothing happened . I waited more than 24 hrs (3 days ) and it still isn’t working . It is still waiting for activation . What do I do to fix this ?