Default 3g disconnects while on phone call

i installed android 4.4 on my i9100, and found a strange issue - 3g disconnects while i'm making a phone call. this issue didnt occur on previous android versions (from 2.2 to 4.3).
what i tried to do:
1. tried adding "ro.ril.enable.dtm=1" to build.prop - did not help
2. tried flashing modem I9100XXMS4 instead of I9100XXMS2.

any suggestions?