How can connection speed be rationed? If it can?

Assume: dial up connection

Assume: someone (ISP and/or telephone company) wants to ration bandwidth.

Question: Which of these (ISP local server or local telephone company), if not both, can limit/restrict a dial up connection to a single kbps baud rate?

I'm asking because with recent updates to IE, ISP browser, and telephone grid...where I once was able to get connection speeds at 40+ kbps, (suddenly one day and since) I am presently only able to connect at a single speed: 31.5 kbps. Formerly, if initial connection was at low (say, under 37 kbps), I only needed to refuse the connection/speed (i.e., cancel the connection) and then dial in again until a satisfactory speed was provided from the ISP Service); but no longer...as connection is always the same (31/5 kbps) no matter how many times I request a connection.

Because the loss of connection speed and speed refusal option occurred at about the same time as the above changes (updates and grid upgrades), it's difficult to eliminate any one cause. Calling the ISP or phone company doesn't help much as the question goes over the head of answerers...or they try to convince me it's my modem/modem setting controls; however, unless modem design has changed significantly, I've been led to understand that connection speed setting (baud rate) is not modem selectable...modems will accommodate the current pass through speed up to their rated limit (56kbps).

Would like to have an informed answer to this dilemma so I can "go after" the right party to fix the problem (and the diminishment of service).

Thank you very much for your expertise and assistance.

UG