I was on 4.2 build .191on my c6606 from T-Mobile. And I found a leaked Rom for 4.3 and I decided to flash it with flash tool and after I booted up I got the same problem so I checked my apn and it was configured to a pcworld thing and I didn't have the option to add a new apn so I clicked restore default and it fixed it self. The problem was gone and my phone worked all the way with calls. Only problem I had was that it didn't show my apn for T-Mobile and my data connection was gone as well as mms but sms still worked. Give it a try.
I also returned back to my original Rom because I couldn't get LTE nor mms. Hope it works.