Most of the time when I see flights on either flightradar24 or flightaware it doesn't matter whether you use the 2-digit IATA code or the 3-digit ICAO code, the flight number is the same, i.e. a flight will be listed as UA1234/UAL1234.
But some airlines show different flight numbers for IATA and ICAO. Just picking a couple of random flights I can see on Flightradar24:
SunExpress flight XQ231/SXS5J
Pegasus Airlines flight PC398/PGT35R
Turkish Airlines flight TK656/THY3YE
I've also seen this on RyanAir (FR/RYR) flights. Why do the IATA and ICAO identifiers have different flight numbers?