Do you have another line so that you can still get calls?
If so, just leave the phone off the hook. Put something over the mouthpiece so they can't hear what's going on at your place.
Maybe blow a police whistle in their ear. I don't know if you should warn them or not - it could damage their hearing.
Have somebody sneeze in the receiver - that would require sanitizing afterwards, though
Can you have someone do baby talk.
How about have someone gargle or sound like they are going to throw up?
Try to get as much information out of THEM as possible. How are you? How is your weather?
IF you have the time, that is.