For future reference when you have a service visit set up sign in to My Account as the PRIMARY Comcast user account (email address). The appointment will be listed in your account. Take a screenshot of the page. (If you don't see it immediately check back since I don't know how long it takes to show up online).
The day before the appointment take another screenshot. Then do another one after the close of the business day (for this one it is important to have the time and date in the screenshot)
Make sure you answer your phone when the automated system calls to remind you of the appointment and when/if the tech calls to let you know he is on the way.
If the appointment is listed after the close of business day and the tech is a no-show you are entitled to the $20 credit.
Then check again early on the day of the appointment. Screenshot and save it if listed.
You can also use this against Comcast
From the FCC regulations Installations, Service Interruptions and Service Calls
http://www.fcc.gov/encyclopedia/evolution-cable-television#sec11
"No appointment cancellations are permitted after the close of business on the business day prior to the scheduled appointment. If the cable installer or technician is running late and will not meet the specified appointment time, he or she must contact the customer and reschedule the appointment at the convenience of the subscriber. "
Cable providers generally ignore "at the convenience of the subscriber".
