There is a great deal of controversy on how to handle partials, the situation when you can not hear the full callsign of a caller. Perhaps it is due to QRM or you, commendably, are trying to dig out a weaker signal. For whatever reason, you can only get the first half or the last half of the call.
Of course, the best thing would be to say "JA1?" and have eveyone whose callsign does not start with JA1 would stop transmitting. Sadly, that is simply not the case. Maybe half the pileup stops calling but there are likely to be some W5s and K9s and F6s, calls that are anything BUT JA1, who keep right on calling. It's as if those people hear "JA1" and they say to themselves, "He can't hear that station so maybe he will now hear me." Bad thinking but whatever the reason, it's a real problem.
First, you can make a case for not calling the JA1 if that's all you can get. Choose another station. If you hear JA1X, that makes it easier to justify calling him.
There are cases that can be made for choosing any one of several responses.
Send "JA1X 5NN" That tells the pileup that you heard the JA1 well enough to assume that a quick QSO will result. That seems to quiet down the pileup better than saying just "JA1X?"