https://www.bt.com/help/account-and-billing/manage-account/thinking-of-leaving-bt
if someone left a few days before the actual expiration of the minimum term , or a few days after ( and therefore pay a few days at the higher out of contract price , those few days are likely to be a very small amount of ££, a more important thing to consider is the billing period is not likely to coincide with the date notice is served, or the date of leaving, so chances are the next full month will be paid , and the days charged for after the cessation are refunded, so don’t cancel the direct debit …..trying to leave on an exact date is not always possible but it’s hardly worth stressing over a day or two either side of the date you want , but obviously to make the last month of the minimum term and the 30 day notice to be the same period call as close to a month before the expiry of the minimum term .
The problem here is that BT don't understand the concept of giving notice. There is no legal basis to insist on phoning 30 days prior to the contact ending since there is absolutely nothing wrong with ringing 45 days before to provide the notice of cancellation for the 5th Jan as the contract is not being cancelled early since it will run for its full term.
At least 30 days notice is required to quit , to take that to a ridiculous length, someone could start a 24 month term and once outside any cooling off period, serve notice for service to cease in 23.5 months and expect that notice to be held and not acted upon until the 23 month ….although it may not be necessary to wait until around a month before the expiration of the minimum term to serve notice , it’s the path of least resistance and hardly an imposition……as far as what the representative said about a ‘fine’ for serving notice earlier than 30 days , that’s probably a poor choice of words , if notice to quit were given 45 days in advance but it was stipulated that the notice wasn’t to start for 15 days , there is a chance that the notice is processed immediately and 15 days ETC is automatically raised , better to be cautious and serve notice around the date that’s likely to get the wanted outcome , rather than arguing later that the notice should have been held in abeyance until 30 days before the minimum term expires.