To change your greeting, go to PBX Settings then click Greeting. Once you click on "+" sign, you can edit the Main Greeting or Add New Greeting by uploading an mp3 file you've already recorded or you can write a script and an automated voice will announce it.
You can create your own MP3 greetings using smartphone apps. Search app store of your smartphone for "mp3 voice recorder" and install one of the apps you like. Once you create your greetings on your smartphone share/send it to your computer as MP3 file. You can upload MP3 greetings at My Account /PBX Settings / Greetings link.
You can also use your computer's microphone to create your MP3 greetings using an opensource free software such as Audacity