The only BT calling plan that seems to match your description, Jennyjoan, is 'Friends and Family International', where you pay £2.11 per month for discounted calls overseas.
For calls to the USA, both landline & mobile calls are then charged at 6.27p per minute (plus a call set-up fee of 24.32p)
For calls to Australia, the charge is still 6.27p per minute to landlines but it's 37.62p per minute for calls to mobiles. (The set-up fee also applies).
https://www.bt.com/landline/calling-costs