/usr/lib/x86_64-linux-gnu/libgsm0710mux.so is in libfsoframework-dev 0.12.0-5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgsm0710mux.so.3.0.0
/usr/lib/x86_64-linux-gnu/libgsm0710mux.so.3.0.0