I do this, too. Shut the phone off, don't return calls/texts that aren't necessary. I even created a filtered folder in my email so that I don't get alerts from new stuff in my inbox if my phone is on. When I'm headed down that hole, I usually answer a few calls and things from people I talk to a lot and just mention that I'm getting really swamped or something, so that they have a reason in mind when I stop reaching out or responding for awhile.

I don't know how else to handle it - the isolation is the only thing that feels semi-OK. And it sucks, because then it reinforces the depression, but I feel awful and the depression feels worse if I force myself to socialize - like I pay later for socializing & putting my depression off, if that makes sense.