I have the same question. i signed up to try out Sonic Internet and Phone first and now ready to cut the cord with cable and do Sling TV. Sales person said they had a "deal" with Sling TV for Sonic customers. I cannot find this deal on either the Sonic website or the Sling website.
BTW, I did sign up for the week free with Sling to try it out with my Smart TV (works fine) and thought I would get a notice of some sort to sign up with a discount code since I'm a new Sonic customer. This did not happen.
And, I do not recall the deal exactly but it was definitely a reduced rate from what is published on the Sling website.