T46S+BT40+Plantronics Voyager Legend - audio static on some calls

[BillYea]

T46S+BT40+Plantronics Voyager Legend

Typically the bluetooth headset works well.
On some calls, user complains of audio static. Volume on phone is 50%. Volume on headset is 50%. I estimate only 5-10% of calls have audio static.

T46S less than a year old: firmware 66.82.0.30 hardware 66.0.0.128.0.0.0
T46S is plugged in with AC Adapter.
PBX is FreePBX 2.9.x / PBX in a Flash

Not sure where to look or if this how BT headsets usually work.

thanks for ideas