My cat Pigeon and I were separated for a while, she lived w/a friend, and when she came home, she paced the house and yowled like that for a while, a couple of days at best. Once she got used to being back at home, the yowling stopped. I would think, though, that if your cat was missing a companion, he would've yowled right from the start. You say that you call to him when he yowls but do you actually go to him, pick him up and check him over? When was the last time he was checked out at your vet's? I think maybe I'd feel better if I were you by taking him to my vet and having him thoroughly checked out. I will say, however, that my Siamese Coco Puff can be very vocal and he walks through the house sometimes vocalizing but certainly not for hours at a time. I also have a deaf cat, Creamsicle, who does that on occasion when she's looking for me and she's exceptionally loud because she can't hear herself. One last thought: are you sure that your kitty is male? Females will do that when they're in heat. I wish you well and hope to read that your kitty checked out just fine and that he just likes to hear his own voice. Prayers going up for your kitty and you.