Ah, you had a secret handset, not (land)line (or, as per Ernestine, instrument).
Basically, decades ago, 411 was the first "N11" to come into common use, for "local directory assistance"; and 611 was for "repair service". But all 8 were reserved ( https://www.nationalnanpa.com/number_resource_info/n11_codes... ), and there was no way they were going to rescind all the already-existing phone numbers with "11" in them, or even with "911", since they'd been in use forever.
Some random examples of real numbers with "911" in them: 800-345-1911 is the South Hackensack office of TransAxle; 800-349-1100 is (or was) Louisiana Health Monitoring.