Whilst I rather enjoyed this animation, I felt that it wasn't half as good as it really could have been.
It seems as if the main joke that you're trying to get across here is the sudden drop of his accent to a more "lower class" accent after the hypnosis part. It's as if you made the animation expecting that sudden change in his voice to get some laughs, but really, it wasn't that funny. I certainly didn't laugh. Perhaps it was just poorly executed, but I get the impression that this sudden drop implies that Theodore is not who he seems. He seemed up until this point to be a bizzare, eccentric, and extremely creepy man with a dracula complex. Characters who are as extreme and wacky as this are often seen as being funny. However, the sudden drop of his accent to something else implies that it is all an act, which is no longer wacky and unworldly, because people do put on accents and answer phone calls in real life. Hell, you did it for the entire length of this video, so I think you'll agree that it plants the whole experience firmly in the "mundane" and stops it from having that whole "twilight zone" weirdness that you were doing such a good job of establishing in the earlier portions of the video.
The other joke that you try to get across is the whole hypnotism thing. To be honest, it felt to me as if you had simply run out of things to say by that point and were trying to insert some padding to your prank call. Did you never have a childhood? There are a thousand better ways to stretch out a prank call than going off on a bizzare monologue such as a hypnotism sequence, which only actually tells the person on the other end of the phone that you are trolling them. It's too obvious, and at that point it loses all humour because you KNOW that the person on the other side of the phone is no longer convinced. It's like any other practical joke of this type - when the 'victim' no longer believes the joke, it is no longer funny, and you might as well just stop. The 'hypnotism' portion of this just wasn't very funny and reeked of poor scripting.
However, the first few seconds of the animation are very good and your choice of music is a perfect fit. The "Yeeees, this is Theodore. Who rings?" line was just excellent and the delivery actually raised a smile on this end. The way how he calls it a "telephone system" instead of just a "phone" is the sort of creepy, slightly unusual character trait that you should have built on more heavily, instead of the Jeckyll and Cockney Joe voice change that you did at the end. His low, lazy, drawling voice is excellent and gives an impression of him being in control the whole time. However, as the animation goes on, it gets less funny with each passing moment, sadly. It appears to peak early.
However, it was still, overall, a good piece with something original to it that was well worth watching, and I wouldn't have written this lengthy review if I didn't think it was worth my time or yours.
TL:DR version - Music good. Character good. Voice good. Animation passable. Sudden accent change bad. Hypnotism bad. Brock bad.