You ask whether posting an ad and having a phone conversation is enough to get you arrested for solicitation, but what matters is what you actually say and not the forum in which you say it.

Solicitation is an offer to exchange sex for money.  If your ad doesn't do that and you don't make that kind of offer on the phone, then you haven't broken the law. If you do offer sex for money then you have, and if this was a sting the police can arrest you at home or anyplace else they can find you.

Keep in mind that an offer of sex for money can be more subtle than just "Ill f*** you for $350."  Even if the offer is made more subtly and discretely, if a jury would understand your meaning beyond a reasonable doubt then you could find yourself with a conviction.

And if you do get a conviction you'll need to be sure not to repeat yourself again.  In many jurisdictions a first offense is no big deal, but subsequent offenses can be much more serious.