Overcharged bill for 6 straight months

What else can I do if I am being overcharged?!! I am extremely frustrated with AT&T. I was offered a promotion in December for a “free phone” as I was a new customer. I had Verizon at the time and the only reason I switched over was because of this promotion. I LOVED Verizon and am regretting switching over as I have been lied to for months. I was told I would be credited back in January as my bill was overcharged. I then spoke to someone in February since nothing changed. This continued on and on and is still an issue today! I was also told NUMEROUS times that my bill would be lowered, and it hasn’t. I am at a loss with AT&T and feel like I am being taken advantage of. I never leave reviews for any company or product but would tell anyone to stay away from AT&T as they cannot be trusted or relied on. I am looking into switching back over to Verizon and completely still at a loss of what to do.