I agree with musicallinguist, but you can get this information for a given corpus if you really want to. One option is a tool called Phonetisaurus, which is one of a family of statistical graphone tools. Its intended function is grapheme-to-phoneme conversion, but one of the data files it builds along the way is a set of grapheme-to-phoneme mappings attested in the input corpus, weighted according to the computational model. It attempts to deal with the issues of context/cooccurrence that musicallinguist noted. This would be relatively tricky unless you're pretty familiar with the programming, and probably more effort than you'll want.