You can create a new conversation by doing the following.
First, click on the plus icon next to the search bar in the top left of the chat view.
You'll be led through a series of dialogue boxes.
If the number doesn't already exist, you'll be prompted to enter contact details for the new contact.
Once you've filled in the contact's details, you can also add marketing data such as the source of the contact and which job location the contact is to be associated with:
If you wish to start a new conversation with an existing contact, but under a different phone number, you can follow the same process but at the "Create a Candidate" step, unselect the "Create a New Candidate" option in the top left. Instead, you will be prompted to select an existing contact to start the new conversation with.