Not get offered promotion. Billing problem

It is so frustrated during the past seven months. Today I accidentally got to the forum and saw so many people have similar situations.  Hope they resolve their issues, and hope I can get help from this forum.

Seven months ago my phones were with other carrier. At the end of my call for internet question, the xfinity agent asked which company is my mobile carrier, and if I switch to xfinity mobile I will get $100 for each line. I have two lines, so I will have $200 promotion if I switch to xfinity mobile. She confirmed a few times that i will get $200 credit. So I switched two lines to xfinity mobile.

I have called them many times and hope they can apply the credit to the billing statement. Each time they said they talked to the back office and I will get the credit next month / in a few days etc. I waited for next billing cycle and nothing happened. I called again and same story happened again and again. In the first few months, I thought it may take a few months to get credit so I was patiently waited. After 4 / 5 months, I still did not get the offered promotion, and I realized they seems use the same strategy, give you the hope but not really solve the problem. 

A few days ago I called again,  one agent gave me another phone number and said that department will solve the problem. When I called the phone number, all machine answered and then they ended the phone. I did not get a chance to talk to anyone. 

Please help.