I’m not sure why the OP feels compelled to answer every call even if he has an elderly mother with many health problems. It’s highly unlikely that someone calling about his mother will have “Unavailable” as their caller ID.
My complaint about some of the allegedly frum robocallers is that they’re breaking the law by not disconnecting when I hang up on them. If I pick up the phone again after hanging up, they’re droning on about the sale I don’t care about or the Chinese auction that I’m not interested in. G-d forbid that I should actually need the phone for an emergency.