Not too many, is the short answer!
A compatible handset, an up to date operating system and your data settings configured correctly, are the requirements.
Configure your data settings by following this guide: https://www.plan.com/apn-settings
The service is only available in the UK.
Calling is unrestricted¹ and call duration is decremented from your inclusive tariff allowance of minutes per calendar month.
¹Charges will be incurred by calling chargeable numbers as they would be by utilising the cellular network.