In reply to:

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If he has call waiting on his phone, or 1571 (?) then it might be worth checking the dial tone to make sure that it is the standard one or that he hasn't any messages

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I just thought it would be worth mentioning that if 1571 is enabled and you have a message you will get an intermittent dial tone. To get round this make sure the 'wait for dial tone before dialling' option isn't checked within the Dial Up options.