Using the trusty mobile phone usually is best cause ladies love to hear our dulcet tones for various reasons and they have a mobile number for their own security/peace of mind and they can check the number against Ugly mugs-whatever that is sez you.
Using PM's is far less effective unless you and the escort already know each other or you have a lot of reviews/activity on the website so that they can evaluate you.
Even then some escorts can go days without checking their messages.
Stick to the mobile phone as nearly all of them request at the start of their profiles.

:sadwavey