In some cases, a user may try to dial out to a PSTN outstation, but the call fails. The outstation is then placed in a 'Retry Pending' state and will automatically redial after a certain timeout. If during this time that particular outstation dials in, instead of terminating the call directly after the end of the incoming transmission, the connection time will be based on the type of connection that was initiated from ClearSCADA.
For example, if you had requested an Open Line, and that dial attempt was waiting for a retry after failing when the outstation dials in, ClearSCADA makes the assumption that you wanted the Open Line you had requested and so keeps the line open. Similarly, if you did a One Shot, it would only keep the outstation connected for a short period of time and then disconnect.