Hi Bubcomet! If you can't find the number or if the lines are busy, I recommend contacting them via social media or chat. They will have to escalate the process; they don't process the refund immediately during the call. You can use the following link to request a call, contact them via chat, or see which social media platform you're using.
From there, you can request the number of months you've been overcharged. They'll ask you to send proof or images of the charge.
I believe they have a maximum of two business weeks to issue a refund.
https://support.google.com/googlenest/gethelp?sjid=14958904963902314317-NC
I hope this helps.
Rwiinbo
