I have had the same problem The cause was Security Master's do not disturb settings. If you use Security Master you may find that Messages is marked with Do Not Disturb. If so, untick the box.
Re: Number of Rings before Voicemail
I've also just used this method. I was a bit confused by all the x's and y's as my voicemail number is simply 121 but it worked perfectly and now runs for 30 seconds
Cindy,
Yours was the only suggestion that worked., This error message started this afternoon and I spent 2 hours trying everything. I don't know where you are but if you ever get to the UK there's a drink waiting for you