Some numbers are normally charged outside of your call or text allowances, so if you have a zero spending cap, you won't be able to call or text these numbers at all. These include:
most free phone numbers (calls to 0500, 0800 and 0808 numbers are free from a landline, but may not be from a mobile phone)
numbers starting with 08/070 (these are usually car insurance, banks, RAC etc.)
calls to directory enquiries
premium numbers (usually starting with 090), for example, for competitions of reality shows like X Factor
international numbers or texts
texting shortcodes for things like competitions or buying ringtones.