How can I set up a voicemail notification for customers, if they call your Text only number?
When a customer calls into a text-only enabled line, they will get Twilio's generic "Number is not available" message. Since this call is not actually connecting to your Zendesk account, you cannot tailor your message to alert the customer of why they cannot get through to your support team.
However, if you enable the Text number for Talk, you can then route the call to an empty group so that those calls will go to voicemail. Once this is setup, you will be able to tailor your voicemail message for your Text number, to inform the user why they aren't getting through and which number they should be calling to get support help from agents. For more information on group routing, see Routing incoming calls to groups of agents.
If you would like to create a text message every time someone calls in your number, see How can I send an automatic SMS when customers call in?.