One other thing--with the Voicemail Tone set to on (I think) it may take a little while for the light to clear. If it hasn't cleared within 30 minutes, it probably won't. The Uniden manual isn't super helpful--here's what it says about "Voicemail Tone":

Voice Mail Tone sets the voice mail tone. If your service does not support SDT
message signal, set your VMWI (Visual Message Waiting Inculcator) not to detect
SDT message signal.
1) Press the MENU soft key. Select the Global Setup menu, and then the Voice Mail
Tone Submenu.
2) Press volume up or volume down to set the
voice message indication (SDT message signal) to On or
Off.
3) Press the OK soft key. You will hear a confirmation tone.

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Point is, I believe that with "Voicemail tone" set to ON, the phone is supposed to go off-hook now and again to see if the msg waiting light should be flashing or not. What I don't know is what triggers it to go off-hook. Obviously, if you pick up the line, you'll be able to tell if the stutter dial tone (SDT) is there or not, one would think the phone could, too.

I think that if you set the Voicemail Tone to OFF, the Uniden phones are supposed to look for the FSK signal from the ATA (which it should send). You might try the above test again with Voicemail Tone set to OFF. No guarantees, though.