"Best effort" is below rigorous QoS specifications though. Saying "99.999% of calls must complete" with a rigorous definition of completion is different than saying "best effort".
Most things in life and law are not rigorous. Not everything is low-level systems engineering. If the law was "99.999% of calls must complete" someone would find one that didn't complete, then buy several phones, make that call over and over, and sue the company.