I think being "On Call" for certain hours is a little problematic ...
Firstly customers may be in (very) different timezones, and perhaps neither of us fancys talking about vector calculus at 3 AM
Secondly there probably won't be many interested people (at first) and so I would have to wait for many hours just to get one job.
And thirdly, I may need some time to actually come up with a solution, depending on the difficulty of the task
But I could imagine creating an E-mail-adress, where people can send their problems (for instance as a short text or a document), I look into them, and if I have a solution, I reply to the mail. Of course a real-time-chat would also be interesting, but I don't think that you would need video, so I'd be willing to do a live-text-chat for people who need some broader explaination of a certain topic.
In both cases I'm not entirely sure how to handle the price and BTC-transfer ... I totally understand that people don't want to pay a stranger for a solution, they didn't see yet, but it's also depressing to work on something, present the solution and not be paid afterwards
So probably I would charge some money beforehand and if the customer is happy with my work, he can pay a bonus.
Let's give it a try:
All readers are welcome to PM me problems, and the first 2 of them get a free answer (if I have one of course). I'm very curious, what's going to happen ^^