Being a Saturday afternoon, I would suspect it is telemarketers.
It's a program they use that multi-dials a large volume of numbers at the same time but doesn't have the operators at their end to connect with more than a small percentage. As a result, your phone rings, you answer and no-one is there. That's because other people answered first and did manage to get put through. They just keep redialling your number along with everyone else they missed until they finally catch you.