/usr/include/spandsp/private/ is in libspandsp-dev 0.0.6~pre20-3.1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/spandsp/private/adsi.h
/usr/include/spandsp/private/async.h
/usr/include/spandsp/private/at_interpreter.h
/usr/include/spandsp/private/awgn.h
/usr/include/spandsp/private/bell_r2_mf.h
/usr/include/spandsp/private/bert.h
/usr/include/spandsp/private/bitstream.h
/usr/include/spandsp/private/dtmf.h
/usr/include/spandsp/private/echo.h
/usr/include/spandsp/private/fax.h
/usr/include/spandsp/private/fax_modems.h
/usr/include/spandsp/private/fsk.h
/usr/include/spandsp/private/g711.h
/usr/include/spandsp/private/g722.h
/usr/include/spandsp/private/g726.h
/usr/include/spandsp/private/gsm0610.h
/usr/include/spandsp/private/hdlc.h
/usr/include/spandsp/private/ima_adpcm.h
/usr/include/spandsp/private/image_translate.h
/usr/include/spandsp/private/logging.h
/usr/include/spandsp/private/lpc10.h
/usr/include/spandsp/private/modem_connect_tones.h
/usr/include/spandsp/private/modem_echo.h
/usr/include/spandsp/private/noise.h
/usr/include/spandsp/private/oki_adpcm.h
/usr/include/spandsp/private/queue.h
/usr/include/spandsp/private/schedule.h
/usr/include/spandsp/private/sig_tone.h
/usr/include/spandsp/private/silence_gen.h
/usr/include/spandsp/private/super_tone_rx.h
/usr/include/spandsp/private/super_tone_tx.h
/usr/include/spandsp/private/swept_tone.h
/usr/include/spandsp/private/t30.h
/usr/include/spandsp/private/t30_dis_dtc_dcs_bits.h
/usr/include/spandsp/private/t31.h
/usr/include/spandsp/private/t38_core.h
/usr/include/spandsp/private/t38_gateway.h
/usr/include/spandsp/private/t38_non_ecm_buffer.h
/usr/include/spandsp/private/t38_terminal.h
/usr/include/spandsp/private/t4_rx.h
/usr/include/spandsp/private/t4_t6_decode.h
/usr/include/spandsp/private/t4_t6_encode.h
/usr/include/spandsp/private/t4_tx.h
/usr/include/spandsp/private/time_scale.h
/usr/include/spandsp/private/timezone.h
/usr/include/spandsp/private/tone_detect.h
/usr/include/spandsp/private/tone_generate.h
/usr/include/spandsp/private/v17rx.h
/usr/include/spandsp/private/v17tx.h
/usr/include/spandsp/private/v18.h
/usr/include/spandsp/private/v22bis.h
/usr/include/spandsp/private/v27ter_rx.h
/usr/include/spandsp/private/v27ter_tx.h
/usr/include/spandsp/private/v29rx.h
/usr/include/spandsp/private/v29tx.h
/usr/include/spandsp/private/v42.h
/usr/include/spandsp/private/v42bis.h
/usr/include/spandsp/private/v8.h