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The secret to a successful ringtone

Phone company Nokia has launched a competition for people to create a new ringtone for their mobiles, to replace the classic Nokia tune.

What makes a ringtone catch on?

For a start it has to be "really irritating" says musician Thomas Dolby, who co-invented the ringtone synthesizer embedded in Nokia phones.

He revealed all to James Coomarasamy on BBC World Service.