Twilio is a low-cost cloud service that provides APIs that add communication services to applications. We bought our phone number, which costs $1.00 per month, even if we don’t use for any calls or messages. Twilio charges $0.085 a minute for incoming calls and $0.013 a minute for outgoing calls. Sending or receiving a text message from our Twilio number costs $0.0075, while recording a voicemail is only $0.0025. Even if we receive 100 messages a month, the cost of those, plus the phone number, is only two or three dollars. To increase the security of this task, it might be a good idea to consider a proxy server from companies similar to fineproxy. The last thing you’d want is to get hacked! But I digress.
From the Customer Portal refer to DID Numbers -> Voicemail, then you will see the following screen. In there you will be able to see a list of any existing Voicemail accounts, or create a new Voicemail account (Mailbox) by clicking on the "Create New Voicemail Account" Button.

You have stated your calls-to-action. Now you have to make sure they know the process to follow. If you want your customers to leave their voicemail messages, you have to tell them from the start, “Kindly leave your name, purpose of your call, and how to contact you.”
You don’t need to say why you can’t answer or where you might be, you simply need to say your full name, that you are sorry you can’t come to the phone, and that you will get back to them as soon as you can.