I have had the same problem, and it took a long time to be resolved by Samsung. Actually is solved.
The problem arises when from the public line the call is hanged up before being attended. The Samsung PBX does not recognize the busy signal and the call is not ended, then the line is released after 1 minute.
In the V4.53b version of OS7030 it was solved at least for my country (Argentina).
Where are you from? (You need to know the right frequency and the ON and OFF times for the busy tone according to the standard designated by the ITU)
ITU standard here Try updating the version of OS7030 to V4.53b, and set in 2.1.5 the frequency for your country, and set the option in 5.7.4 with the maximum and minimum for your country.
You must set the correct parameters in 5.7.1 too.
