You can file your complaint anytime if you are not happy with AT&T wireless or internet services. AT&T complaints department is available 24 hours at your service to resolve your issue. For AT&T wireless complaints, you need to call at 0800 331 0500. If you have AT&T internet complaints, you can use the AT&T complaints number 0800 288 2020.
You can file a dispute to ATT using this ATT Complaints form:
AT&T GNS United Kingdom, 80 Victoria Street, London, SW1E 5JL, United Kingdom
If your complaint is related to AT&T internet services, you can contact AT&T business complaints department at the same AT&T customer complaints number 0800 331 0500. However, if your complaint is regarding AT&T business internet, you need to call at AT&T corporate complaints number 0844 920 0689.
As such there is no AT&T complaints email address but you can use social media to write your complaints like Facebook, Twitter etc. AT&T complaints about customer service will be entertained quickly as customer satisfaction is their priority.