Hi Todd,
First the UC320W system does not support call-waiting on an analog line(s).
Second the UC320W has Voice Mail as a feature of the system. The MWI light on each phone is controlled by the UC320W's voice mail system.
Please login the UC320W and check Status -> Voicemail and look for new messages. After clearing any new messages the MWI light should go out.
Note: the MWI will be illuminated if a shared VM account has a new message.
Note when used with UC320W the phone MWI light is not related to a Verizon (or other provider) system voice mail service. Completely independent. With the UC320W providing the VM service, please cancel or disable the VM service on your analog line(s). Any stutter dial tone on any analog line will cause outbound dial problems (UC320W checks for solid dial tone).