This is often because doctors aren't paid unless they physically see you in the office. Current insurance payment policies do not recognize phone or internet consultation as billable events.*

My doctor provides this service and does so as follows:

I call my doctor leave a voicemail(Name, DOB, Phone#). A "triage nurse" (their exact wording) returns my call. I describe my symptoms and answer a few questions, these are then relayed to my doctor. If he feels the issue doesn't require a visit but requires a prescription the order is placed electronically and a nurse calls back to confirm my desired pharmacy, provide instructions and answer any additional questions.

*While some companies are making efforts to amend these policies, this holds true for the majority in my experience.