Most plan.com tariffs come with inclusive, unlimited allowances for minutes and texts, to STANDARD UK numbers, located in the UK at the time of activity. 

If you have incurred charges for calls, the likely reasons are:

• You have exceeded our 3000 mins per calendar month Fair Use Policy. Click here for full detail: https://guides.plan.com/fair-usage/
• You have made a call(s) to a chargeable service, such as a competition number or premium rate service. Such services are not 'standard' and may not be based in the UK.
• When roaming abroad, dependent on location, you made or received a call(s).
• You made an International call(s). An International call is a call made from the UK to a number outside the UK or a call made back to the UK from a Rest of World destination.

To prevent reoccurrence of charges, from your my.plan dashboard you can block access to any calling activity that might incur charges.

Go to:

CONTROLS > PREMIUM & ROAMING > PREMIUM CALLS

Manage all devices from the column header options or choose any user(s) and toggle ALLOWED  to BLOCKED.

Full itemisation is also available from your my.plan dashboard, through the USAGE & CHARGES tab.

Not sure about a number that's on your bill? Check details by clicking on the link: https://psauthority.org.uk/for-consumers/ive-been-charged-what-next