You can create different voicemail messages for each status that you can set your extension too e.g. Away, DND Holiday etc. To record and set these messages first log into your 3CX control panel for you extension found here https://warminsterschool.3cx.co.uk then navigate to Settings > Greetings. Once there click on the Phone button as shown in the image below.

You will then be presented with a dialog box to name your voice mail file 

Give the file a unique name, in my example I will leave it as sample then click OK.

You phone will call you and you need to follow the instructions to record your message.

One you have gone the the process of recording your greeting and saving this via the phone options You can then select this greeting for that status in the screen shot below I am picking the sample.wav greeting for my away message