Believe it or not, but some people don't want to be tied to a computer, or they don't have a BB.

If that's not suitable to you, there are plenty of gals who do only email, or sometimes both.

over e-mail, others prefer e-mail over phone, some have no preference.

Email is more convenient for me, leave a message when I am able, get a message when she is free.  I sense the phone only is more quick...  a quicker set up...  or not.  Some girls are hard to reach since (hopefully) they don't answer when they are with a client.
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The ladies might prefer:
a phone call versus email because maybe they are more of a phone person
they do not require appointment setting
might not have access to a computer on a regular basis
some agencies do not have a website and they prefer a phone call which is a lot quicker than email to them
Every lady works differently I do not list my phone number I prefer email as initial contact or through my website.When I had my number listed there were times I could not pick up the phone but could return an email fairly quickly.
Just my .02

For some ladies, it is challenging enough for them to log on, and then to post an ad. After that, they don't want to bother with the internet. Their choice. Sometimes, they get a friend to post an ad for them.

I'm guessing with the Agencies, they probably get a lot of spam (publish your e-mail, and you are flooded with spam). If you send them an e-mail, they might miss it. Less likely to waste the lady's/agency's time with a phone call than numerous e-mails that lead nowhere.