Okay, I confirmed that VOIPo-specified ports are forwarded.

I also tested per fisamo's instructions. In both cases (Voicemail Tone set to On or Off), when I have a message waiting & I pick up line 1, I get the stutter tone. The Line 1 light on the PAP2 blinks slowly while I have that line off the hook. If I delete the message via vPanel, nothing changes as far as the Line 1 blinking on the PAP2 goes -- still blinks slowly. The phone, however, goes from dial tone to busy signal, as one might expect when the phone is off the hook. The PAP2 Line 1 slow blink only stops when I hang up line 1.

The first couple of times I tried it, the MWI light went off after I deleted the message via vPanel and put line 1 back on the hook (i.e. hung up). However, subsequent tests using the same procedure did not work (MWI light remained on). There is no stutter tone when all messages are deleted and the MWI light is still on.