I don't know who she is and I have no idea where she got that number from.  She must have written someone else's number incorrectly.

It makes a difference where you are.
Lying to LE is a crime in many places, especially federally.
LE are not total imbeciles.
He probably has your phone records already from the last nine months, so if you've never heard of this person, but have talked with her a few times, then zippo you've committed a crime right there. She may have provided details already of your meeting and place, if there was one.
Best answer is to ignore the first few calls, then if this persists, says nothing except that he has to talk with your attorney and you have absolutely nothing to say.
That will end it because you cannot be compelled to give a statement that possibly could incriminate you.
You may or may not need a good white collar criminal defense attorney.
If this LE comes knocking on your door, invoke your right to counsel and send him away immediately.
Do not get dragged into a conversation.
I'm sure the earlier posters mean well, but it's not so simple.

what should i do?

just kidding. this call sounds real. i'm pretty sure that's how law enforcement works, they call you on the phone and ask you if you'd mind incriminating yourself - "hey, we heard you might have robbed a bank or two. we've got lots of evidence, but a dictated admission of guilt sure would help speed things up, whaddaya say?"

Same thing happened to me.  Turned out to be her ex who had access to her computer causing problems for both of us. Do not take a chance though, spend the $$ and talk to an attorney and have him ready to speak for you.

This may be a prank, but if it's an LE investigation I'm certain she's aware and might have suggestions as to a good attorney.

There's no reason to say anything!!!! Just hang up and if they call again hang up again.  When they come to the door and show their badges, THEN tell them you want to retain your 4th and 5th ammendment rights and ask for an attorney. I'm no attorney, but I think that's good legal advice anyway.

Don't fall for it!

So they got your number from a provider, or from her "trick book."  So what?  Maybe she even gave a statement implicating you.  Again, so what?  We're talking about a chickenshit misdemeanor at best.  At least in California, a misdemeanor arrest REQUIRES the arresting party to witness the criminal act.

Even if it really is LE calling (which it might not be), think about it.  Do they have any evidence that you yourself broke the law?  Did they videotape any of your sessions with this provider?  Did they witness it with their own eyes perhaps?  If they did, don't you think they would have busted you right then and there instead of calling you on the phone later?  

Clarence37 is right on.  Cops can get a lot out intimidation if you let them.  Don't be a sucker.  Can they arrest you for "remaining silent?"  They probably busted this poor girl, found her trick book and are now fishing for statements to bolster their case.  Or maybe they're just having a little cop fun, ruining her business and scaring her customers.  Remember, they're sadistic pricks.

This also goes for you saps who are cornered by the fuzz on your way to your car after leaving her apartment.  They flash their badges and tell you you're in deep trouble if you don't make a statement, but they'll go easy on you if you cooperate.  Again, think about it.  What do they have on you?  So you left her apartment, so what?  You could have been a Jehovah's Witness having a Bible Study with her.  

Don't lie to them, but don't let them browbeat you into saying something you don't have to.  What are they going to do if you respectfully decline to answer their questions?  Also keep in mind that any statement you give can be used by them as "probable cause" to justify all manner of mischief against the girl in question.  Do you want to be a part of that?

It may sound easy to talk this way in the safety of an Internet forum, but I've had cops try to scare me into giving up a provider before.  I didn't talk, and guess what?  I didn't go to jail and they didn't call my wife, although it was strongly insinuated that both of those outcomes were likely if I didn't do the "smart" thing.  Anyway, that's my two cents.