Annab

Hi, there, I'm new to this forum. I'm studying german by myself and there is something I never encountered before which I don't understand and I have no one else to ask. The lesson is about the answering machine and it's the following: Sie waren selbst nicht da, aber sie wissen, wer angerufen hat. Why do they use the infinite? Wouldn't be better saying "sie sind nicht da, aber sie weissen wer angerufen hat.