Customer Service won't honor promotion on phone

I ported over two numbers, one received a $500 off phone promotion, and the other received a $400 off promotion (both over 24 months). Both phones just finished paying off, and I only received $249.96 in promotional credit off the first phone and $283.22 off the second phone. I called customer service, and after going through 4 people, the lady literally laughed and said they were "unable to assist" with this, refused to connect me to anyone else for assistance and suggested I "google corporate escalations" because she was "told in a meeting it was on the website" but couldn't give me any further assistance. I am sick that I have been cheated like this! I called in the beginning of the payments and was assured that the discrepancy would come off at the end of the payments--but obviously it didn't. Very, very disappointed with Xfinity!