Avoid persistent misuse
You need to make sure that you do not fall foul of the Ofcom persistent misuse rules.
The principal requirements of Ofcomâ€™s approach to silent and abandoned calls are:
â€¢ limiting abandoned calls to a rate not exceeding three per cent of all live calls made in any 24 hour period;
â€¢ playing a brief information message giving details about any call answered before an agent is available;
â€¢ Calling line identification (CLI) information to be provided on outbound calls, so that consumers may return the call;
â€¢ a 72-hour period before a telephone number receiving an abandoned call may be called again without the guaranteed presence of an agent; and
â€¢ unanswered calls must ring for a minimum of 15 seconds.
I'm not sure that there is a total number - but I would expect that any more than 3 attempts (space 72 hours apart) could be viewed as misuse.