How can I get ATC to recognize aircraft callsigns during flight?  I assign a callsign on the per-flight loading screen, but except for rare occasions, the only ones that consistently work are with the freeware Boeing 737-800 151 livery package. I've tried adding callsigns to the atc airline of various aircraft cfg. files (as per how it works with the 737-800 pkg., which the sim ATC does recognize), but nothing seems to work.

Any thoughts on how to make this work would be greatly appreciated.

Thanx.

Just because you add them to the aircraft using the "Details" section does not mean you will here ATC call you that.  Do you have EditVoicepack installed?

No. I do not know what that is. What I dont understand is that if ATC recognizes all the callsigns from the B737-800 151 liveries addon pkg, why wont it recognize others. As mentioned earlier, I've tried with others, & on very rare occasions, it has worked when I've added them, but I cant figure out how to make them work consistently.

FSX has many callsigns built in. Others have to be added to the in program voicepack. It's been awhile since I messed with it, but it matters where and how you alter a cfg. file. Letter case (capitalization) matters. Google "Editvoicepack" and check it out. In the past when I spent way too much time in FSX, I built my own airports  and airlines, Editvoicepack made it so that ATC used my airport names and my airline name. Kind of fun. There is other software to make your own flightplans, also.