It is really not good to keep enzymes frozen at -20C.
I red somewhere it has something to do with properities of salt solutions at
Either add 50% glycerol and -20C (does not freeze).
or add 10% glycerol, snap freeze and keep at -70C.
phosphate buffer is not good for freezing anyway.
Especially slow freezing - it will give unstable (too acid?) pH as one of
the salts can precipitate.