Fry took the copy she was offered. She saw references to traffic noise, bird song, a barking dog. And there was a puzzling reference to a loud, echoey voice, like someone shouting in the background, but inside a building – and not really shouting as such.

‘Do the boys in Technical Support never go to church?’ she said.

‘Sorry?’

‘The shouting they refer to would be the eulogy for the deceased councillor, delivered by the vicar from his pulpit. That means the funeral service had already started when the call was made.’

‘It has to give us a better chance of identifying him, if we decide to follow that route,’ said Hitchens.

Fry sighed as she recognized one of the phrases the DI used to avoid commitment.

‘They’ve done well to bring out that amount of detail from the tape,’ she said. ‘I didn’t notice any of it. The quality of the recording is too poor.’

‘But how is he disguising his voice?’ asked Hitchens. ‘Don’t you need special electronic equipment to do that? How did he manage it in a public phone box?’

‘I’m coming to that,’ said Petty. ‘Technology makes it a lot easier these days. He used a telephone voice changer. Probably something like this –’

She produced a tiny aluminium device no bigger than a pocket watch, with a couple of buttons on top. It looked almost like a miniature computer mouse.

‘This is a voice changer?’ asked Hitchens sceptically.

‘It has six voices to choose from. You select the one you want by using the button on the casing. Then you simply hold it over the telephone mouthpiece and speak into the microphone on the top. There are more sophisticated devices on the market, but for most ordinary purposes this is sufficient. You can pick one up for less than twenty pounds on the internet.’

‘It’s easily small enough to carry in a pocket,’ said Hitchens.

‘Certainly. There’s even a little chain, so you can attach it to a key-ring.’

‘And most people would take it for a garage door remote.’

Petty laid two small handheld dictation machines on the table. ‘This one has a tape of the original phone call in it,’ she said, pressing a button. A familiar voice filled the silence, metallic and vibrating with artificial echoes.

Soon there will be a killing. It might happen in the next few hours.

‘That’s enough, I think. Now listen to the second recording. I borrowed this from one of my colleagues.’

She pressed the ‘play’ button on the second machine.

Soon there will be a killing. It might happen in the next few hours.

‘What do you think?’ she said, turning it off.

‘They sounded identical.’

‘If we’d recorded a longer piece, you would probably have noticed the difference. Actually, the second voice was mine.’

‘You’re kidding.’

Petty held up the voice changer again. ‘It was pretty close, wasn’t it?’

‘Close? It was uncanny.’

Petty passed the voice changer round.

‘How is it powered?’ said Fry.