I've only ever had one telephone interview for a helpdesk job in the past - at least I think it was an interview. Was sort of like "How long have you been using computers?" and "What Operating System do you currently use at home?", then was asked in for personal interview.
Not sure about this whole telephone interview business. If I'd done one and was SURE I'd messed it up, I'd probably brew a nice big fart towards the end, and when parting goodbyes go "Okay, yes, have a nice day, you too, bye! <drop handset to rear > *PPRRRAAAAAPPPPPPPP*."
At least that would amuse me for a while.