Billed for phone number "change"

I decided to switch to Verizon only after being assured I could keep my previous phone number. The day after my service began, I found that my number had changed to a number hijacked from a Comcast customer, who proceeded to call me multiple times demanding that I straighten the matter out. After several phone calls, my number was restored. Imagine my surprise to find a $28.11 charge for a "phone number change" when I checked my Verizon account online. I called and was given credit, but then an email came stating it may take a couple of bills for the credit to appear. So in order to not be deliquent and disqualify myself for the promised bonus for paying my first two bills on time, I will be forced to pay for Verizon's mistake. Very much regretting making the switch.