This could be due to any number of reasons. Do you have a QoS plan set up through your router? If not, look up how to do so in order to prioritize VoIP packets over regular data packets. Are you using Google Voice, or another service provider? Is the PBX system (voicemails, ivr, etc.) located in the cloud, or do you have a PBX server in the office (Asterisk, FreePBX, etc.)?