About a decade back I was writing embedded firmware for a telephone for the hard of hearing. We ran compliance tests to ensure the handset was compatible with T-coils. In the early days of say Bell 500 and Bell 2500 sets, the coil in the handset speaker was big, and naturally emitted enough for hearing aids to pick up. As speakers shrank, the size of the coils dropped, it takes active design analysis to make a handset that will work. IIRC, ANSI has a standard for this compliance, this link leads me to believe it was updated in 2019: https://ieeexplore.ieee.org/document/8906258