Incorrect Xfinity mobile bill

I recently switched to Xfinity because they offered a promotion for their 300M Internet service at a monthly price of $35, along with a free line of phone service for 12 months. Unfortunately, I've encountered an issue with my mobile bill. The free line of phone service hasn't been applied correctly, and despite contacting customer service multiple times, I haven't been able to get it resolved. This situation has been quite frustrating, as I've invested a significant amount of time trying to get assistance without success. What steps can I take to address this problem, considering the lack of effectiveness from the company's customer service?