I'll be honest I'm not totally sure what you mean by this?
I'll be looking at the code in the near future, I've just got a major upgrade to simcom X to box off first.
A little bit of information from my side (off the top of my head)...
ESJFS gets the data from either the JoinFS whazzup file, FSX/P3D via SimConnect or through FSUIPC/XUIPC.
It puts together what Euroscope needs to see for a flight plan from the data gained and sends it to EuroScope.
The callsign is one of the bits of data it gains.
The arrival and departure airports are another.
So see if they are set and showing in the JFS whazzup file or in the ESJFS debug window.
The only instance of no callsign that I know of is when FSUIPC is used to gain the data.
I'm not sure why this happens but it only happens when FSUIPC is used.
When this does occur I told ESJFS to use the aircrafts squawk code instead.
This is so aircraft can still be identified and also adds realism as most Radar scopes I've seen show aircraft by squawk and not by callsign (although they can be code callsigned..., opening up a can of worms and going off topic....).
I'm also not sure how you are initiating handovers?
I've not written anything to do that and each EuroScope client is essentially not talking to each other.
As far as I'm aware (with the exception of area), radar handovers are normally conducted over the phone Controller to Controller.
Example: (Hello Edinburgh Radar its Glasgow Radar.
Do you see that 1234 squawk east of DeanCross tracking 090 degrees? Well that's GABCD, inbound to you....).
If you can pinpoint the issue a bit more (does it only happen using FSUIPC?) and let me know how your conducting your handovers(do you get a message in EuroScope when somebody tries to handover?) I'll have a look into it.