I thought I had the answer somewhere? they do have voicemail.
I have a phone with a blinking Hold key, not sure if there is a mailbox associated with this phone( customer doesn't speak English very well...)

Message waiting! If no voicemail then phone to phone is set. Intercom #3 and dial extension number. If unknown do the procedure for each extension. Do this from the phone with the light!!

OK, I thought it was that...I had her try her own ext, still blinking...I will have her try them all...Thanks

I think the blinking HOLD light was a static charge issue...unplug the set or reset the phone system.