Working as a prostitute in private is not an offence, and neither is working as an outcall escort. Nor is it illegal for prostitutes to sell sex at a brothel provided they are not involved in management or control of the brothel. Street prostitution, however, is illegal.
Escort services are illegal, when the escort services serve as the middle man and arrange such sexual activities. The idea behind this is the sexual activities to operate with a certain level of anonymity.
Escort services are legal to offer escort directly and arranged direcly between client and the person offering escort without middle man. People can communicate and decide what to do and what to offer.
Many folks assume an escort service is a same as prostitution because money is involved, but there is a critical distinction. An escort can provide as well personal companionship that's of a non-sexual nature.
You have some higher end clientele that will pay top dollar to get dates with people who are able to talk about very specialized subjects or able to spend time on some remote island for 72 hours and know how to order section things on the menu. It gets that specific.
It becomes a criminal act if sex is involved for financial or commercial gain. It doesn't matter if it is your typical back-alley prostitute or a prostitution on a higher end, usually that kind of prostitution associated with escort services, what makes it illegal is the payment of cash or other things of value for sexual activity and sexual services.
Catching escort services that are participating in illegal acts is not as easy these days, but some of charges could include conspiracy to commit prostitution and solicitation.
You don't just look in the yellow pages to get these escorts service, especially in this day and age when technology such a useful tool, both for law-enforcement and for people that operate the services, so there is a certain exclusivity.