It's difficult to do things by phone because there's literally no "paper trail." That's why lawyers use letters, and insist on getting things in writing so they know who represented what and who's accountable for their representations.
The IRS, like everyone else, makes mistakes. You've for your signed compromise, so you just need to enforce it effectively. Send them a copy of it, along with your certified copies of your payments to the address on the notice you got. Use certified mail (or FedEx) so you can prove their receipt.
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The OIC being received is not the same as it being accepted. But it is almost impossible to tell what happened for sure. The OIC might not have included all the years of liability. There might have been a default on the payment, which could be a mistake or not. You should contact a professional in your area to look at your situation. Assuming that the IRS is just not honoring the OIC is likely a mistake. There might be a valid reason for it, and there might be a mistake by the IRS.