It’s quite impossible to detect the IMEI, the simplest solution for you would be the operating system. For example, Android, BlackBerry, iOS, Windows Mobile, and therefore serving content for them exclusively. I’ve done this multiple times in my items so it’s not hard to achieve, but targeting the individual IMEI and hardware specifications is quite impossible as these are constantly changing throughout years so doing this would result in an obsolete site in a year. I recommend targeting the operating system.