YOur answer has been asked by some one else, may be u look at the other queries and u will find the answer over there as well.
u will have to arrange 200 lines of PSTn for ur 2000 user on 10:1 ratio or .1 erl traffic.
On PSTN line gives u 64k of bandwidth. I am not sure about the other part of ur questionas the reduction of bandwidth will completely flop the ISP.
May be some one else can come up with a better answer.