Small business 2 lines. One has been disconnected erroneously

My business partner recently divorced and moved. I purchased and took over the business, including 2 business lines and his, other carrier, cellphone. I wasn't here when it was connected but my wife was. They hooked up both lines  (VOIP) Internet. The tech said one line would be on a carrier number and it would show up different on my bill but if you call this long existing business number it would ring thru as normal. I checked this and it worked fine.

8 or 9 months later it rang and answered disconnected. One of my customers let me know and I know it had been working until fairly recently. Some of my customers use this now canceled number and some use the primary number, which still works. Have talked to 4 reps on phone and had 2 techs come to house and they can't explain what happened, how to fix it, or even find that number in their system. I looked it up on ipqualityscore.com and it shows valid and carrier to be at&t South Central Bell.

I need this number back. Has been part of the business for o er 20 yrs. Any help would be appreciated.