Silly Question but...

[holmes4]

I have a standalone Caller ID box (it speaks the ID) that flashes when there's a VOIPo voicemail. As stevech says, many Panasonic cordless phones (though not mine) will do this too.

[BarbJaw]

So basically any phones on the market that are based on DECT 6 protocol will tell you in some form if their is a new voicemail on the VoipO server?

[stevech]

DECT 6 doesn't assure that the handset has a message waiting light. But I'll guess that the majority do, except for the super cheapies.

The light (LED) is often on the top edge where it can be seen at a distance. And the display screen on the handset probably displays message waiting as text or a graphical icon.

The message waiting scheme is well standardized for all phone systems. The phone line from the wall, and the VoIP ATA to which a phone is connected - passes message waiting on/off data messages. A message is sent to turn the light on/off, by VoIPo or by the phone company's office in the case of POTS wall jacks. The POTS systems often repeat the light on/off messages a few times an hour. This allows a phone to be powered-on and "catch up" with the status. I don't know if VoIP systems repeat like this. I also guess that the better phones "remember" the light status across a power off/on.