Number not in service receive iMessage?

I text somebody and it sent as an imessage (via iphone), but tried to call a few days later and got 'call failed'. Tried the same person on somebody else's mobile and it said the number is 'not in service'. Would the imessage have sent even if the number wasn't in service?
Does number not in service mean they've changed their number? Or could it be that their phone is being fixed? I know nothing about phones (obviously!)
Thanks to anyone that answers!

Added (1). I definitely know the number is correct. It's somebody whom I thought was a good friend…

Maybe that wasn't even a number. Try checking it again.

Number not in service means just that. They have probably changed their phone and taken a new number with it.