Website: https://www.att.com/support/smallbusiness/article/smb-digital-phone/KM1195027/
Assign the number you want to forward calls to. If you have chosen Always Forward Calls, click Save.

Having experienced multiple "small business phone systems" (AT&T hard wired system - OK, AT&T SynJ wireless system - TERRIBLE, and a vTech system - almost as terrible) and for basically the same price plus a little bit of configuration time, the Grandstream products worked better, had more features and had much better sound quality than the others. I hope this helps. I bought this system for my office due to the affordable price. It initially had some problems with calls cutting out and poor call quality. I was going to return it, but the quality improved for a short while. The problems came back a week later and I had my phone guy look into the phones, wiring, and telecom installation. He said everything else was good but there was a problem with the phones. We hooked up a different system and viola! No more problems.
With German-engineered sound design, the new Snom series is designed to provide the small business market with the best sound quality and make it easier to add new people and integrate employees who use their mobile phone more than their desk phone. The new platform can start with just 2 lines and 2 phones, then easily grow up to 100 lines and up to 300 cordless, desk phones, and smartphone mobile users. And it’s your choice to have your phone services hosted in the cloud or installed to traditional landline phone lines in your closet — whatever makes the best financial sense for your organization right now.