Hi Seguima,
You're correct in that with Ribbit you pick up the cost of the forwarded call. For example, if I left you a 3 minute voicemail, 3 minutes would be deducted from your monthly allowance.
You're also correct in that had you been using Vodafone voicemail you would not have been charged for me leaving the message. However, Vodafone would charge you 3 minutes (from your monthly allowance) to call 121 and listen to the message. Now here's the benefit of Ribbit, you won't need to call to listen to your messages because you can read them instead. If you wanted to listen it’s easy to play the mp3 that was sent to you by email for no cost at all!
So in this example, regardless of whether you used Ribbit or Vodafone Voicemail, you'd use 3 minutes of your monthly allowance. But with Ribbit you’d have the convenience of being able to read your voicemail.
David
Vodafone Costs:
http://online.vodafone.co.uk/dispatch/Portal/appmanager/vodafone/wrp?_nfpb=true&_pageLabel=template10&pageID=OS_0030&tabIndex=1