Since the 081 craze began, not always easy to tell which network a number belongs to. Many people cannot reconcile the new gen prefixes with a particular network. A number with the 0817 and 0818 prefix could easily be Etisalat or Airtel. In the era of the 080’s and 070’s, things were a bit easier.
Hopefully, this would lessen the confusion, the accidental airtime overuse and help you navigate the 081 craze.
PS: Remember Mobile Number Portability became effective about a year ago. You might want to consider the possibility that some numbers prefixes may have ported out of their corresponding networks. The chances are pretty slim though.