Do I get an online voicemail?

Yes, regardless of whether you have a cloud telephone system or not, you get a free mailbox that can be customised with personalised greetings. You can even play different greetings depending on the phone status and decide whether callers can leave a message or not.

You can find instructions for telephone numbers under "Setting up an answering machine (mailbox)", for the cloud telephone system as part of "Setting up call forwarding in the cloud telephone system".