voicemail no longer working after shortening ring time

I changed the length of time before going to voicemail from 30 to 20 secs.  Now if it rings for that time the caller gets a message to say the number is not available.  If I dismiss the call before 20 secs it goes to voicemail correctly.  Live Chat were worse than useless, all I got was a link to set my voicemail up and a repeat of the same link when I told them it was already set up.

Hi @fauvehound .  Did you get your issue resolved?  I have the same problem and have got nowhere with Live Chat.  Could @Kevin PM me as well please?

When someone calls and the phone is not answered, it rings for twenty seconds and then a message says number not recognised and cuts the line off. 

When receiving a call, your mobile is automatically set to ring for 30 seconds before it's diverted voicemail. However you have the ability to change this at any time to a choice of five-second intervals, ranging from 5 to 30 seconds. All you need to do is enter **61*123** followed by the number of seconds you'd like your phone to ring for, then # and call. So, if for example, you'd like your phone to ring for 20 seconds, you'd need to dial **61*123**20# and press call. Once you've done this, a message will pop up on your screen confirming the update.

These are the instructions I followed as did the staff in the store, I have also tried turning vm off for 24hrs and back on - no joy