tjubb's link just led me to a parked page, maybe the domain name has expired? I found this site and it worked well; no nasties or hidden stuff on the site as far as I can tell - they're just offering a service in the hope you'll subscribe for the premium.
AMR is quite common in mobile telephones. However, AMR has limitation in file compatibility. We have to change to to other file formats that have have high compatibility. To , it will be better to find a tool to help you.