and 162 for example although a third terminal 20 is set outside the communication network system 1 it still can selectively dial the extension numbers 122 142 and 162 of the terminals 12 14 and 16 in the communication network system 1 to communicate via a public switched telephone network pstn it should be explained that the extension numbers 122 142 and 162 of the first terminal 12 the second terminal 14 and the fourth terminal 16 are different to each other however the common group extensions 120 140 and 160 of the three terminals are identical additionally the first group extension 120 the second extension 140 and the fourth group extension 160 are standard for the pbx server that is to say the first terminal 12 the second terminal 14 and the fourth terminal 16 need not extra support of the pbx server please refer to fig1 together with fig2 fig2 illustrates a flowing path of signals among a caller a callee and a pickupper according to one embodiment of the invention in the embodiment the third terminal 20 represents the caller the first terminal 12 represents the callee the second terminal 14 represents the pickupper in order to further understand the concept of the invention examples in the following are described in detail.