Look into the log from the plugin. If the message isn't received at all then there is probably nothing we can do. The protocol here is very simple and the plugin shows the characters that have been received from the device.
I have Vonage and do not get reliable vmail notification.
Wireless phone adapters more than likely also will not work.
Correction: Just remembered that I did increase the "sound quality" on my Vonage account from "normal" to "high". I'll test further when I have time.
Looked last night at the MLNetCallerID log and there were no messages about voicemail. That seemed odd to me as the voicemail indicator on my Uniden phones correctly shows vmail or lack thereof. Was going to investigate further today.
And voila...with no changes on the computer, the phone wiring or anything...this morning I had a voicemail left and suddenly the indicator changed in ML. Looked in the NLNetCallerID log and sure enough there was a message indicating I had voicemail. When I cleared the vmail, another entry appeared in the log showing that I had no vmail.
Would be curious to know if other Vonage users had this miracle occur overnight. Guess we'll have to wait to find out if Vonage fixed something or this is just a random time when it happens to be working correctly.