i have a bt line, if i dial an 0808 number, do i have to pay for the call?

0808 is a freephone prefix just like 0800.

So unless you have a very unusual landline tariff then they will be free.

Not always the case with mobiles however.

right, ok. thanks for your reply.

In fact, hardly ever the case (most mobile companies only allow free freephone calls to specific charities and the like).