Yes, 0800 prefixed telephone numbers are completely free to call from a mobile phone on the BT Mobile network just as they are free to call on any other network in the UK.
What are 0800 numbers? What are freephone numbers?
Much like 0808 numbers, 0800 numbers are freephone numbers – they have been free to call from a UK landline since they were first introduced in 1985 and have been free to call ever since. Mobile phones, however, are a luxury-turned-mainstream development of the last 20 years and as such have only just been synchronised with BT’s traditional landline standards. Since 1st June 2015 every landline and mobile phone in the UK is bound by law to not charge a penny when someone calls any UK freephone number be it an 0800 or an 0808 number. Calls to freephone numbers are cheaper than 01, 02 and 03 numbers which are charged at standard (local) rate unless you have spare free minutes in your contract or bundle, because freephone numbers never cost a penny and don’t eat into your free minutes allowances.