Usually when people reach me it is a big problem, very hard to solve. I have 20 years of experience in SIP, Asterisk, OpenSIPS, FreeSwitch and VoIP. So in this case I'm not only charging per my time, but also for my experience. The maximum amount of time in a troubleshooting section is 4 hours. If the problem is not solved I will not charge.
Qualification: I only engage in troubleshooting if the customer have a clear idea on how and when he considers the problem solved. I do this to avoid situations where a lot of time is spent and the customer is not satisfied with the solution. I prefer to know in advance to avoid conflict.
Sessions are taken usually using Team Viewer. I can also use another customer preferred tool.