My old dog used to howl if the phone rang and you were in the house but hadn't heard it (usually if I was up in my room, listening to music, in the shower etc). He never did it if nobody was home, as I had been for a walk once and as I approached the house I could hear the phone ringing but Dexter ignored it. Useful old boy
My friends dog howls whenever he hears a song that she has or has had previously as her mobile phone ringtone. He even does it when he hears songs she had a couple of years ago!
It is a very eerie and sometimes a sad sound, I think.