I've asked providers when they were planning on providing incall in the near future.  This doen't seem to be a problem with providers.  Of course it feels more comfortable and easier to ask them after you've seen them already, but I've asked the question to providers who I had not yet seen and it didn't seem to be an issue.

If this is really a deterrent, I suggest finding providers with permanent incall locations.  This should allow your hobbying to fit in with your schedule.  

BKMan

Since I can "only" do incall, I have found that providers who do offer "incall specials" from time to time have been open to e-mails I've sent them asking about when they will be offering these specials again. I've even asked them to notify me in advance. Works for me...And hopefully for them too...

Just asking is fine, by phone or email.