Facebook Audience Best Practices for formatting Phone Numbers

Phone Numbers - When using a Phone Number attribute for custom audience, remove symbols, letters, and any leading zeroes. You should prefix the country code if COUNTRY field is not specified. 

E.g If you are using a leading '+', this does not receive any matches when hashed. The format must be, for example, 447123456 and not +447123456. This formatting has an impact on the match rate when using this as part of single or multiple attributes.  

Further information on Normalization for Multi Attributes can be found here.