A VoIP communication infrastructure may need to serve a substantial number of users with an acceptable voice quality. This paper investigates the suitability of the Asterisk PBX server to provide VoIP communication capabilities with an acceptable Mean Opinion Score (MOS) quality to a large number of users. The blocking probability metric is used to measure the capacity of the PBX server while MOS is used to assess the quality of the voice calls. Analytical results show the suitability of the Erlang-B model in characterizing the capacity of the Asterisk PBX server considered in this work. Experimental results have shown that the Asterisk PBX can effectively handle more than 160 concurrent voice calls with a blocking probability of less than 5% while providing voice calls with average MOS above 4.