Back in the dial-up days once call waiting was introduced you were routinely instructed to disable it. I had a box sort of like the fax switcher that I purchased on ebay that would instead listen for the call waiting tone and the box itself would ring, allowing me to pick up the phone attached to the box and receive the calls. It solved the problem of being able to use the phone line for hours on end for internet downloads without losing the ability to actually receive calls.
I also found if I identified a spam call fast enough I could hang up and switch back to the dial-up connection before it was dropped. I had to be real quick but I could usually avoid being dropped from a Starcraft match.