Clarification on Teams Audio Conferencing costs

I'm trying to get some clarification for a customer of mine, who has some concerns about costs incurred for the audio conferencing PTSN lines. I tried checking documentation but couldn't find an explicit answer on this. 

Is the cost per minute charge for the use of an audio conferencing PTSN done on a Per User basis? Or is it simple when the line is used, regardless of the number of users called into it? For obvious reasons, this is a point that makes our customer wary as they don't want to incur multiplied costs during some of their company-wide meetings if the per minute charge is multiplied for every user utilizing the number. 

Thanks in advance!