We used to get loads of these - several per day - and about an equal number where, on answering, there was only silence. There is a huge "industry" of scammers, using autodialling of huge numbers of targets simultaneously. If you answer such a call and there is no rogue immediately available to talk to you, you get the silence.
Our cure is a phone system that filters incoming calls, rings on those that come from numbers in the memory, and diverts all others to a message that allows the caller to respond or to leave a message.
The result of this is blessed relief: zero spam calls. Our phone is from BT, but I'm sure other makes are available.