In addition to what you stated, the IRS has no need to sue to collect unpaid taxes. If all the letters they send are ignored included those sent via registered mail, they have enough authority in the IRS regulations to simply freeze your assets and place liens against them.
Same here. They called. We do Call Screening, and didn't pick up. I don't pick up the phone to unknown callers. They did not leave a message. If they don't leave a message the first time they call, I put them on Call Block. The caller is unknown. Skrew them. Up theirs. Bottom-feeders.