Yes, you will need an IDA number, which could be an 0800 number or an 03 number (0800 numbers are totally free for your staff to call from UK landlines or UK mobiles and 03 numbers are generally included in people's bundled/inclusive calls).
Once set up, you would provide this number to your staff members for them to dial 'through'.
They would be prompted to enter the number they want to call, and a pin (if you set one).
You would then be billed for the duration of this call (unless you have enough minutes in your tariff to cover the call)
The cost of an IDA number is £19.95 one off setup fee, and £4.95 per month to run, plus the cost of your chosen tariff.