I've just seen a recommendation for telephonenumbers. It is called: E.164 from the ITU-T (TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU)

Due to copyright issues, I must not post it here. But if you want additional input, you could contact ITU and ask them for a copy.

Best regards,
perl -le "s==*F=e=>y~\*martinF~stronat~=>s~[^\w]~~g=>chop,print"