Seems to call about 3-4 times per month. Who is this?

I was getting daily calls from this number leaving no message. I tried to answer one and got dead air. The caller ID said "SOT ELECTRONICS". I checked with them and they report that they are seeing someone spoofing their caller ID from several numbers. See the Wikipedia page on "Caller ID spoofing" to see how easy this is to do. Since I answered one call (to dead air) and then contacted them by email about a week ago, I haven't received further calls.

I get at least one call a day from this number.  They always hang up after 3 rings