Are you talking about BTYahoo unmetered access or something else. BT themselves don't do dialup internet access.
It's to stop people staying connected 24/7 and most ISPs do this. There are a limited number of connections available for 56k dialup so the limits are imposed so everyone gets an equal share of the equipment.
Do you have a PlusNet dialup account, just curious why you are asking about BT services on this forum?
Here are the terms and conditions for BTYahoo! dialup access available via the www.btyahoo.com website::