I know that gambling is forbidden for a Jehovah's Witness. Would this include a phone in competition on the radio, for example to win money or a holiday?
I am one of Jehovah's Witnesses.
We don't gamble because:
This does not include a radio contest because the other radio listeners in the contest aren't losing any money if they don't win the prize.
With regard to your follow up question, "would a football team of Jehovah's Witnesses be able to participate in the coin toss to decide which end they started from?" - Yes they would as long as they weren't doing something similar to what is mentioned in this paragraph taken from a 2011 Watchtower: "For example, auspicious numbers are chosen for lottery tickets; the uttering of certain words is forbidden among superstitious mah-jongg players; and a puff of air is blown over dice before they are thrown. Why? Gamblers often believe that luck will, or at least might, influence the outcome." (http://wol.jw.org/en/wol/d/r1/lp-e/2011167)
Here is an article that gives more details on how far to take the prohibition: http://wol.jw.org/en/wol/d/r1/lp-e/2002808