To enable default or set custom voicemail welcome message from inside the 2ndLine app:
From home the screen, tap burger menu icon on left.
From the options, tap Voicemails.
On the Voicemails screen, tap on the gear icon on the top right.
To enable the voicemail feature, please turn on the Use Voicemail button.
Below that, you will see options to preview (and select) Default Greeting or do your own Custom Recorded Greeting.
The + button next to Custom Recorded Greeting will enable you to record your own version and set it as a welcome message.
PS - You can turn off anytime, to deactivate voicemail.