In most countries cell phone numbers are not geographical. Instead they have their own unique codes/starting digits used nationwide. This is not the case for several countries: Argentina, Brazil, Canada, Mexico, USA and some Caribbean nations. In these countries you can not differentiate a cell number from a landline because they all use area codes.
Only if your friend is in a country where area codes are used for cell numbers, and only if the cell number doesn't already incorporate the area code.
If you were given a cell number starting with zero you must drop this digit.
For example, if your friend in the UK gives you a mobile number of the format 07??? ??? ??? and you are in the US, the digits you will dial are: 011 44 7??? ??? ???.
This website provides complete cellular numbering format information for most countries.