A format built for phone calls, not fidelity
AMR was designed for one job: carrying a human voice down a mobile network using as little bandwidth as possible. It achieves that by keeping only the narrow band of frequencies where speech lives and discarding everything else, which is why AMR files are astonishingly small and why music sounds terrible in them.
That design is why the format survives in call recorders, voicemail systems and the voice memo apps built into older phones. If you have recorded a phone call on an Android handset, there is a strong chance the file waiting for you is an .amr or .3ga.
Set your expectations honestly before you start
Most AMR is sampled at eight kilohertz, which is roughly a quarter of the detail in a normal recording. The consonants that distinguish similar words live partly in the frequencies AMR throws away, so this is genuinely harder material than a voice memo recorded straight to the same phone's storage.
You will still get something useful, especially for a call where you mainly need the substance and the sequence of what was agreed. Plan on a careful review pass, and check every name, number and negation against the audio before relying on the text. Where a recording is important, record locally rather than through the call path next time.
Frequently asked questions
Why is my AMR file so small?
The format strips everything outside the range a human voice occupies, so a long call can fit in a couple of megabytes. The small size is by design and does not mean the file is damaged or incomplete.
Will an AMR call recording be accurate?
Less accurate than a normal recording, because the format keeps only a narrow slice of the sound. Expect a usable record of what was discussed and plan to verify names, figures and anything you intend to quote.
What about .3ga files from my Android phone?
Same thing in a different wrapper. Android often saves AMR audio with a .3ga extension, and it uploads and transcribes here exactly as an .amr file would.