There are several moments during an incoming call that you're able to add announcements.
Once uploaded under this section, you'll be able to appoint them under the Channels tab.
To upload your announcement, choose your file (1) from your computer and submit (1) it after.
The announcement should either have a .mp3 or .wav extension, where .wav is preferred (mp3s will be converted to .wav). For .wav, set the file to 1 - channel, 8000 khz.