>>> A post on a similar site, "whycallme.com" says it is telemarketing for AT&T. <<<
Yes, but that is based on exactly one anonymous comment dated Aug. 20, 2014. The same source also says that the number is within a block managed by "Metro PCS," not AT&T Wireless.
https://www.metropcs.com/metro.html
Metro PCS uses the T-Mobile wireless network, not AT&T's.