Twillio is a valid company - I use them is some of my applications. It's basically a service that lets you program telephone systems (things like "enter you're account number for your order status" or "press 0 to be ignored"). The "sandbox" is a way for developers to test their code. It sounds like someone used your number my mistake or is being a dick and entering random numbers.

I got a call from this number at midnight that was three minutes of some light metal guitar riff. It was actually very nice, as long as it's this time only.