After MP1 coldcalls pf he might have a monster (QQ+, AK) or a medium pp. He might just call pf so as to get another caller and proceed to milk him postflop(SS shove acts as MP1's raise). The ss is all-in and basically out of the hand.
So I call pf and re-evaluate postflop.
I figure calling this shove postflop would be risky(I only beat AK, TT imo) and I havent invested much. Also I will get to see unknown's hand anyway