You can Textline with an existing Signalwire number! This is a manual process for the momment, so we'll need you to send us your Signalwire account information (Phone number, Project ID, and Auth Token). Send those details to firstname.lastname@example.org and our support team will get you set up with a new department using your Signalwire number!
If you are using your own Signalwire number(s) with Textline, you will be paying Signalwire for your messaging costs, not Textline. You will also need to complete Signalwire's regulatory registration on their website.