I had the experience once of having flight 2 and flight 12 and flight 22 all operating on the same route very close together. ATC asked me to amend the flight numbers of at least 2 of them to show the a/c rego instead of number. Then, the next day had flight 2 and flight 22 - close together and same dest. Okay I thought, I will use their rego's instead. What do you reckon the chances of having ECHO BRAVO CHARLIE and ECHO CHARLIE BRAVO ? Oh yeah, it happened. Simple fix but - give me a break .
Pre-COVID I was going to be part of a NavCanada-led exercise that looked at airline flight numbers as it related to comms errors. It was fascinating the data they had for similar sounding call signs. They included a heat map of where and when one flight crew would accept the clearance meant for another flight for every major airport in the country. They were asking the airlines to review the data to find out why these mistakes were happening. In some cases, we could point to a clearance being given right in the middle of a checklist, while in other cases it was just that a flight was too close in numbers to another for the same geographic space, and this likely would have resulted in the airlines changing the flight numbers. It would have been a really great project to be a part of, but then it was shuttered once no one was flying. We pretty much got the data on a Monday to find the project cancelled on the Friday. It was too bad, I was looking forward to that one.
Fast forward to maybe two weeks ago, and I was flying behind our competitors flight on the same route. If memory serves they were late and we were early, but you had two Flight 120s going from the same departure to the same destination along the same flight path, separated by 20 miles. No CPDLC for either of us, so we each just started telling each new ATC unit that we were aware of each other, so we didn't have to have the "similar sounding callsign" conversation every 30 minutes. So I assume that since I've returned to line flying, that NavCanada project has not been brought back.