Sonic.net

I've been receiving calls with caller ID '203' (without the punctuation of course).
When I answer the phone, only silence on the other end, then the line usually hangs up within a few seconds.
I've read somewhere that these are robocalls, but not for the purpose of trying to sell something, rather just a probing of the phone system initiated not from a phone but rather some digital switch somewhere.
Has anyone else suffered the same?

There's nothing special about that ID. You were just lucky enough that your number landed on the list of a someone who didn't program his spoofing system very well.

It could be a probe, but so could a call from 202-867-5309. Many people have reported getting repeated calls with an ID that appears to be a legitimate number, but having the same behavior that you're reporting.