Where to Capture

Hi i am trying to capture VOIP traffic on my office network, i am using a switch, with my router and modem. The ip phones are cisco spa 303 and the switch is intellenet 24 port fast ethernet switch. The phones will ring some countries sometimes, othertimes they wont or the audio is one way. I have used X-lite on my laptop and used wireshark to monitor this traffic it picks up fine and calls complete. Can some one advise where i can plug into network or how i can monitor from one of the devices that are sending traffic to VOIP server but not the device running wireshark.

thankyou for any feedback
:confused: a very confused me