In the Telavox app, below Settings > Voicemail, you can choose which sound is played when you are busy in another call or cannot answer. Below Settings > Notifications, you can choose how you want to be notified. Through email, SMS or why not both?
To record a new sound,, you can either go to My sounds > Upload new sound, or alternatively call your voicemail on number 133 and then press 9.
To upload a new sound, you can either directly upload the sound from your computer, or choose one of the other two alternatives which are: "Call my landline and record the audio" or "Call my mobile and record the audio". When you choose one of these, Telavox will call you and you will be given instructions on how you record the sound. Once you have ended the call, the sound will be added automatically to the list in My sounds.