The more serious the crime, the more effort they put in.  Get yourself an anonymous pre-paid phone and don't worry about it.

They might try and get some info.  What would a provider have on you and why would she roll over if the crime is a misdemeanor?  She might know your name.  Is your name unique?  If not, are they going to call in all 20 John Smiths for a line-up?  

Imagine the prosecution.  She has ads that don't mention sex and she goes up in front of the jury and says her ads are wrong, you did have sex and in return for her testimony she is having her sentence reduced.  There is absolutely no other evidence that sex occurred because the condom was flushed.  What prosecutor wants that case??

Stuff can happen, but it is uncommon.  What tends to be most common involving this is agencies or independents who have interesting little black books.

This is why references are great.  You are John Y. who saw ladies X and Y.  The ladies are kept safe with the system and our privacy is protected.