Welcome to Android Central! Why do you need the IMEI number? Did you bring the phone you were using on Sprint over to AT&T? If so, you can find the IMEI of that device in your Settings>About Phone menu. If you sold the previous phone you had on Sprint to someone else, then you could try contact that person and ask them to read the IMEI to you, but that might be challenging if that person knows that IMEI should be kept private.