AT&T,  AT&T over charging for Health Insurance / Trying to take back Pension Dallas TX Texas

I recently retired from AT&T after 30 years. I was given Health insurance as Group rate. However, I acquired new employment and call to stop Health Insurance on October 1st, 2017. AT&T acknowledge stopping of insurance but continued to bill me (auto pay) until December 2017. after multiple inquires their response (via letter) was we have examined this matter and found in our favor no reimbursement is required 

Pension return. In March of 2018 I received a letter stated that AT&T had miscalculated my Pension payout by $368.36. If I did not repay they would contact IRS and send me to a Collection agency. 

AT&T has just closed the biggest media deal in History. It seems strange they cannot accurately handle the premium payment for insurance and appropriately calculate retirees pension. It is also interesting that both error were in their favor.

I have tried to communicate directly with AT&T but have not been able to get through to them. I am directed to Health insurance coordinator/benefits coordinator/pension coordinator. 

Is AT&T financing their mergers on the retirees’ pensions?