Egon Spengler's Wife?
It was never explained in the movie who Egon made out with to birth Callie but I think I know who the mother or surrogate is.
https://youtu.be/9qTsB3qfNUk?t=74
HE FUCKED THE GOO! Presto. Answer solved. Maybe that's why Phoebe Sue is such a prodigy.