Only receiving 28Mbps and paying for gigablast. other neighbors report the same thing.

You should receive a Cox email when a service appointment is scheduled.  If you don't receive that email, you probably don't have a scheduled appointment.

Have you ever received the approximate speed you expect?  If this isn't just a short-term issue, your area may not support that speed, regardless of what a Cox sales rep may have told you.  If your area can't provide the speed you're paying for, why don't you change your plan to a speed it can provide?  If the speed you're currently getting is within the limit of the new plan, you should get the same performance for a lower price.‡  When/if your area is upgraded, change back to a plan with a higher speed then.

On the other hand, you may have a signal issue.  If a service tech determines the issue is on Cox's side of the demarc, you won't be charged for the tech visit and the tech will escalate your issue to have the signal corrected.  This will require you to contact Cox again and schedule a tech visit to have your signal checked.

‡ This assumes that on a congested node, (a) Cox attempts to provide the full, approximate speeds associated with each customer's plan.  A second possibility is (b) Cox prorates speeds based on each customer's plan.  Upgrading would provide faster speeds, but nobody on the node would get their plan's full, approximate speeds, not even customers on the lowest plan.

I suspect a lot of people with slow Internet assume Cox uses option (b) and upgrade their plan to compensate for the slow speeds.  If Cox does use option (b), then customers have to decide for themselves if the prorated increase in speeds is sufficient to justify the full increase in price.  But if Cox uses option (a), all upgrading their plan would do is increase their Cox bill.

Does anybody have evidence to confirm which method Cox uses?