0500 numbers are used for freephone services which were allocated before 1999.
The 0500 code is always followed by 6 digits.
Like 0800 and 0808 freephone numbers, they are free to dial from a land line, but are usually chargeable from a mobile phone.
More information on 0500 numbers can be found on the 0500 code information page.
Other freephone numbers in the United Kingdom are 0800 numbers and 0808 numbers.