Txtria (one of the SMS API's that Contact Reach integrates with) allows you to convert any existing landline number into an SMS-capable number. In this way, users are able to text the landline number and the same number can send the campaign messages to the subscribers.
First thing you need to do is to convert the landline number inside your Txtria account. If you don't have a Txtria account, you can sign up for one here: https://txtria.com/console/register
Once you're logged in and verified, go to your Txtria Dashboard and find the 'Provision Landline SMS' option in the left side panel then follow the instructions until your landline number is converted.
Once done, copy your Txtria API credentials (click Account then open up API credentials) into your Contactreach Txtria Settings.
Once the credentials are copied, click on Get Number and you'll see a dropdown menu containing the number that you converted in your Txtria account. After the changes, hit Save. You'll then be able to select that number when creating or editing a campaign.