If i set up a BT landline for the internet but change to tiscali landline, will i

[Noodles]

have to pay both? I need to set up a BT line & the minimum line rental for the phone is 10.50 a month. at the same time i want to start tiscali broadband & landline for 15 pounds a month. usually tiscali will cancel my bt line rental for me and swap it over, but i was wondering will they do that if i start up both at the same time. i guess bt wants to charge me 10.50 straight away?