Spinvox has mainly distributed their service through carriers, but will also set up customers directly. Pricing is to be determined by carriers. Their UK prices are here. If you would like to test the service, go to their test site, call the number on the screen, and in a few minutes your message should appear on their blog.
TechCrunch suggests looking at another voicemail conversion service from Jott, that retails at $10 per month for 40 conversions.