Static is generated in the analog segments of the conversation between two parties. With VOIP service, this would mean something is producing the static on the other end of the call (easily determined if static persists with certain call destinations) or something on your end of the call. That something would have to either be your telephone, or any headset device attached to your telephone.
Headsets are easy to isolate as the problem. Just pick up the handset to see if the static goes away.
If you are hearing static in your ear from your own voice, you can be certain that the static source is very local.
Static can be introduced into the analog audio circuitry on your phone via the power supply. If a power supply is getting "dirty" power, then this will likely manifest as static. Some sources of dirty power are light dimmers, motors, microwave ovens, and fluorescent lights.
So try eliminating any of those potential sources to see if the static has stopped. If there is no way to abate the static source, then a power conditioning device should be installed at each workstation that is experiencing local static.