With a public key that exists.
sudo sh -c 'echo "deb-src http://download.telldus.com/debian/ stable main" > /etc/apt/sources.list.d/telldus.list'
wget http://download.telldus.com/debian/telldus-public.key
sudo apt-key add telldus-public.key
sudo apt-get update
sudo apt-get install build-essential
sudo apt-get build-dep telldus-core