The primary reason is that my office is at home, where I multi-task family obligations, which include accommodating the needs of a child with autism. My days are often unpredictable and need to be flexible so I can adjust my schedule around what works best and is most healthy for them.
To complicate matters, my working environment can get ridiculously chaotic and noisy when my youngest is home. I've learned to deal with this, but it would be an embarrassing and nightmare situation while trying to speak to a client in real time. The little amount of quiet time I do get without distraction is highly valuable to me and used to get the majority of my creative work done.
But another reason is that email is an essential part of my organization process. It can get very confusing trying to keep track of everyone I need to follow-up with on a given day...so I rely heavily on previous email exchanges to remind myself where I'm at with a particular project or support request.
I understand that my 'no phone' policy is an inconvenience and genuinely apologize for that. I hope to eventually offer a phone support option to established clients, but this needs to wait until after my family schedule is more consistent.