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GSM to MP3
Make voicemail, call recordings and IVR prompts in the GSM codec playable anywhere by converting them to MP3
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Telephony codec decoded: GSM 06.10 full-rate audio, raw .gsm files and GSM inside WAV are decoded by FFmpeg.
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Standard MP3 output: The result is an MP3 that email clients, phones, browsers and transcription services accept.
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Batch conversion: Upload a whole folder of voicemail files and download the MP3s together.
You can upload up to 20 audios simultaneously.
Bitrate
Determines audio quality and file size. Select 'Auto' and we'll automatically pick the best options for your file.
Telephony audio
What happens to a GSM file on the way to MP3
GSM 06.10 is the speech codec of 2G mobile networks: 8 kHz mono at 13 kbit/s, optimised for intelligible voice at very low bitrates. PBX systems, Asterisk, FreePBX and older voicemail platforms store recordings in it. Almost no consumer software opens the files, so the converter decodes them and writes a normal MP3.
How to use Converter App
1
Upload the GSM files
Raw .gsm files from Asterisk or voicemail exports work directly; GSM-encoded WAV files work as well.
2
Decode and encode
The 8 kHz speech is decoded and encoded with LAME into MP3; the sample rate is kept so no artificial upsampling occurs.
3
Download
Attach the MP3 to an email, play it on a phone or feed it to a transcription tool.
Key features
Asterisk and FreePBX friendly
Files from /var/spool/asterisk/voicemail and monitor recordings convert without renaming or reprocessing.
Keeps the voice intelligible
MP3 at 8 kHz uses the MPEG-2.5 low sample rate mode that LAME handles well for speech.
Small files
Because the source is telephone bandwidth, the resulting MP3 stays small, typically under 1 MB per minute.
Private
Call recordings are sensitive. Uploads and results are deleted automatically within two hours and never reviewed.
Experience & privacy
Built for reliable audio conversion.
Backed by 10+ years of engineering expertise in large-scale data and scientific computing, it’s built for real-world workflows that require uncompromised audio quality, accelerated processing speeds, and reliable, high-level performance.
Trusted by Users
Rated 5 stars on Trustpilot for speed, reliability, and ease of use.
User trust
Academic Use
Referenced in published research and used for interview transcription and qualitative data analysis.
Research use
Where GSM files come from
If you administer or use a phone system you have probably met .gsm files without noticing: Asterisk stores voicemail messages as msg0000.gsm by default, call recording modules write .gsm or GSM-WAV, and many IVR vendors ship prompts in the format because it plays natively on the PBX without transcoding. Older Nokia phones and some dictation devices also used it. The format is excellent for its purpose and useless outside it, since Windows, macOS, iOS and Android media players do not include a decoder.
Technical profile of GSM 06.10
- Sample rate: 8,000 Hz, mono
- Bitrate: 13 kbit/s (33 bytes per 20 ms frame)
- Bandwidth: roughly 300 to 3,400 Hz, standard telephone quality
- Codec type: RPE-LTP (regular pulse excitation, long-term prediction)
- Container: raw frames (.gsm) or WAV with format tag 0x31
FFmpeg contains a complete decoder for both the raw and the Microsoft WAV variants, which is what our converter relies on.
How we convert to MP3
The GSM frames are decoded to 8 kHz PCM and encoded with LAME. MP3 supports 8 kHz through its MPEG-2.5 extension, so we keep the native rate instead of resampling to 44.1 kHz; the result is a smaller file with identical sound. Mono stays mono. The bitrate is the page default of 192 kbps, which is far more than speech at 8 kHz needs but guarantees that no further quality is lost and keeps every player happy. A one-minute voicemail becomes an MP3 of roughly 1.4 MB.
Typical reasons to convert
Forwarding a voicemail to someone's email or messaging app, archiving call recordings in a format that will still open in twenty years, uploading calls to a transcription service such as our audio to text tool, attaching evidence of a call to a support ticket, and migrating prompts from an old PBX to a cloud phone system that expects MP3 or WAV. In every case the MP3 behaves like any other audio file while the original GSM would have required specialist software.
FAQ
Frequently Asked Questions
What is a GSM audio file?
A file encoded with GSM 06.10, the full-rate speech codec specified for GSM mobile networks in the early 1990s. It compresses 8 kHz mono audio to 13 kbit/s, about 100 KB per minute. Telephony software adopted it because it was free of patents and extremely light on the CPU. The .gsm extension indicates raw codec frames without a header; the same codec can also live inside a .wav file.
Will the MP3 sound better than the GSM file?
No. The GSM codec discarded everything above 4 kHz and compressed the rest heavily; conversion cannot restore that. The MP3 sounds the same as the original played back correctly. The point of converting is compatibility, not improvement.
Why is the sample rate only 8 kHz?
Because that is what the telephone network carries. The MP3 keeps 8 kHz rather than upsampling, which would only increase the file size. Every MP3 player handles 8 kHz files; they simply sound like a phone call, which is what they are.
Can I convert WAV files that contain GSM?
Yes. Many PBX systems write GSM frames into a WAV wrapper (format tag 0x31). Upload the WAV on this page and it is decoded the same way; if a player claims the WAV is corrupt, that is the usual sign it contains GSM rather than PCM.
Is there a limit on the number of files?
You can upload many files in one job. They are converted in sequence and returned as a ZIP of MP3s named after the originals. For very large voicemail archives, convert in batches of a few hundred files.
Are call recordings kept on your servers?
Only for the duration of the conversion. All uploaded and converted files are deleted automatically within two hours, transfers are encrypted, and the audio is never listened to by anyone.