I suspect that
VoIP.ms is repeatedly sending SIP Notify to refresh the message waiting state.
If you don't need VMWI (visual message waiting indication), turn off VMWIEnable for the Phone port and the light should stop coming on. You will still have stutter dial tone to indicate a message.
If you want VMWI to work correctly, look for a setting on
VoIP.ms portal, or open a ticket with them (I'm not a
VoIP.ms customer and have no real info on this).