Records Management
Keeping good record keep is a must in the online environment. It will help you when you have to deal with a complaint from the student that you have not been responding to their emails. If you have copies of you responses to their email will help prove your case if it comes up.
I always use email as communication simply for the fact of a record. Anyone who has taught online has had an issue with a student come up. Records of communication are a must for any online instructor. I never give out my phone number to any student, even in my online profile. This directs the student to my email. If a student needs to talk to me directly, they must send me an email with the information they need to discuss and why they wish to discuss it over the phone. This way I have a record. If the discussion over the phone turns away from what was described online, I will hang up immediately and make a note to the administration as to what and why the call was terminated. This way I am proactive in dealing with any problems that may arise from a students behavior.