It may well be cheaper to get a freephone number providing she calls from a payphone or a landline.  Freephone numbers from mobiles are charged for and generally cost around 30p/min.

£1.99 will be the connection charge that is charged for regardless of whether you except the reverse call or not.  Was it BT's reverse charge that your daughter rung or 0800reverse? as 0800reverse is a completely separate company that charge BT and then BT charge you.  At this point BT may add the usual amount on so to make a profit just the same as 0800reverse does!

BT's pricelist for calls made via this 0800reverse can be found here.

Calls to an 0800 number from a BT payphone number cost the receiving company more, you service provide will soon contact you if you start doing lots of minutes from BT payphones.