The elements in solid state protectors are basically two zeners back-to-back so as to give non-polarized (AC) clamping. So a single zener is going to do the same but will be polarity sensitive as you point out.
In many cases the caller will hear maybe a half ring although you may not hear anything.
If you wanted to allow a full ring (or more) then pick up the line you would have to get a little more complex. A delayed relay powered by the ring voltage that would put a short on the line comes to mind.
-Hal
