Almost all consumer phones, like the Panasonic cordless in question, require CLASS message waiting indication like from a Telco CO Voicemail system. Essentially it is sent in the same format as Caller ID in an FSK Bell 202 message on signal is sent to the device, which sees it and turns on the MWI, after the VM is cleared the CO sends another FSK Bell 202 signal to turn off the message light.
I don't believe this type of MWI is supported in the 3300, although some other PBX systems do.