Who called from 7315550149

?

Called. Left no message.

This is the third call we have received in our office from this number. When we answer the call, no one replies on the other side - after  about 20 seconds, the caller hangs up.  The call is repeated either later in the day or the following day

Missed a call at the office while we were on lunch. I didn't know "555" numbers were a real thing....

Yes, just received a call.

Knowing little else about it, I suspect these are related to a social experiment or data collection program. If I knew more about it, I would probably still suspect that.