In general, having the same field (Serial number) to act as several different fields (IMEI on one hand, and ISDN+Sim number on the other) is a bad idea.

You will almost always get stuck at some point when you don't know how to proceed.

My tip would be to separate the data in your instance, and use different fields for different purposes (one for serial, one for IMEI, one for ISDN and one for Sim number). That way you can more easily determine how you want to import new data (and match coalesce).