In morden telephone exchanges, subscriber lines are usually connected to the so-called subscriber line modules. These modules serve both incoming and outgoing calls. An important difference between these two types of calls lies in the fact that in the case of blocking due to all channels busy in the module, outgoing calls can be queued whereas incoming calls get busy signal and must be retry in order to establish the connection. In this thesis, we study the model which takes into account incoming calls`` retrials and find the joint distribution of the number of outgoing calls and incoming calls, special cases are treated.