Today, we have the "SMS Marketing" application which is free (https://www.odoo.com/page/sms-marketing). You will only pay for the SMS you send.

Odoo does not support replies to text messages sent (=Odoo doesn't receive SMS). The idea is not to manage paid conversations by sms (there are emails for that). The philosophy is rather to send a link to a product, information or to your e-commerce website to reach your customer and track him with the link.