The law also covers a promise to provide sexual services in exchange for money/financial gain. So people can be arrested even if money hasn't changed hands yet.
But still, if sex hasn't explicitly been discussed on the phone or by text, then there's nothing to indicate that client was actually planning on having sex with the escort. A client stopped on entering the apartment, or leaving the apartment, could simply be seeking to spend time with the escort.
This is the case with brothels though. The escorts haven't committed an offence if they are in an apartment by themselves.