I have used CallWave for 3 weeks now and in a nutshell here is how it works after you activate your phone by punching in a few numbers:

1. Someone calls you and you can’t/don’t want to pick up (in a meeting, not in the mood, don’t know who it is etc)
2. You reject the call or can’t get to it in time so it goes to voicemail.
3. The service now calls back your phone. Once you pick up, you can listen in to the voicemail that the caller is leaving as they are talking without them hearing you.
4. You can either intercept the call by pressing ‘1′ (like good old answering machines on land lines) or hang up to ignore.
5. You voice mail is now emailed to you as well as you can listen to it from your phone.
6. The cost? nothing apart from minutes that you would have paid if you use the voicemail service from you provider.

My question is, why doesn’t every provider offer this as a standard service??

It should be noted that this only works in the States as far as I know.