Connect to your account on aircall.io, and click on one of your numbers. In the section "Music & Messages", you can activate/deactivate and edit each of your messages: Voicemail, welcome message, absence message. You can as well choose your music on-hold.
Click on the pencil icon on the right of each message and choose how you want to edit the message:
If you haven't got your message yet and would like to record it yourself, you can use online recording services like http://vocaroo.com/, which will let you create a mp3. We didn't include this functionality into our product as the end result is generally of poor audio quality, unless you use a professional microphone. If you'd like to record your own message yourself, we recommend you pay attention to the recording conditions and to the hardware you use.
Do not use iTunes or QuickTime!
Apple's softwares use special codecs for audio recording. Your messages won't play in Aircall. If you are familiar with audio recording, we recomand you to use Audacity and export your audio file in MP3 as descibed here.