While I can't help you with a direct number for Telecom USA, I can tell you that the 101-0987 carrier code is one assigned to MCI or one of their subsidiaries. I can also tell you that Telecom USA is a subsidiary of MCI, and has been so for at least ten years now. I'm not at all surprised that you are getting MCI responses regarding your inquiries about 101-0987.
As for the number printed on your bill to contact Telecom USA, I assume that this is on a BellSouth billing, on the additional pages for your Telecom USA charges. BellSouth probably has you set up such that Telecom USA is a "dialaround" carrier, not your primary chosen carrier on that line, and any billing for 101-0987 Telecom USA calls would be through a page or section of your BellSouth monthly billing, along with BellSouth, your local telephone company, being the primary billing agent or billing contact, on behalf of MCI's Telecom USA.
As for a direct toll-free 800 or 888 type number for Telecom USA, try doing a Google search on Telecom USA and see what happens. I haven't tried one, but maybe you might find something for them with a Google search.