In previous blogs, I’ve mentioned the importance of making a good first impression on a potential client. Most of the time, the first contact will be over the telephone. Since we are often in session, it is very likely that the first time a client hears our voice is on our voicemail greeting. This is why a professional greeting is so important. Identify yourself because you need to reassure the client that they have called the correct number. Clients will be a lot less likely to leave a message if they wonder who will get it. Use a warm friendly voice to say something like, “You have reached xxxx, licensed mental health counselor.” Tell clients what to do if they are in crisis. Although it seems obvious to us, clients may not realize that it could be several hours until we can return their call. I suggest something like, “If you require immediate assistance, please dial 911 or go to your nearest emergency room.” Ask for what information you need. Yes, the obvious name and number, but if it would be helpful to get their insurance information, ask for that, too. Give clients an idea about when you’ll return their call. Common business practice is within 24 business hours. Any extra messages or information you’d like clients to know. For example, if you are no longer accepting a particular insurance plan, the end of your voicemail greeting is a great place to convey that information.
Calls overflow to the voicemail box if they can’t be answered during business hours. This may be during unusually busy periods or if there is no one to take the call.

Any Cisco Jabber calls you can't answer gets sent to voicemail so the person can leave a message. To set up voicemail, you'll need to get your voicemail account information from your administrator.
According to its website, Ngrok is a way to “expose local servers behind NATs and firewalls to the public internet over secure tunnels” which is how we connect the Pi server to Twilio services in two steps after downloading and installing Ngrok on the Pi.