Yes we often say that. Probably someone in your company knows his phonenumber, but just in case nobody knows it, we will ask his number. Moreover, the person is able to call him back immediately after the person finished his duties without looking for the file or something which is put your customer's business card in it. 「念のため、お電話番号をお願いします。」 Or you can also say: 「念のため、お電話番号をお伺いしてもよろしいでしょうか？」 There are some more way to say but your way is correct as well.

this is use in Japanese but this mistake ask for honorific form is " お伺いして”　ask permission way of honorific word is "よろしいでしょうか”　two of hororific word connect 　becoming お伺いしてよろしいでしょうか、his case should be use honorific words Please 's polite form is お願いしします、　This is come from Amrecan English "Telephone number please " but for repects for The US People they say in use the US good company say to same phase as " May ask your telephone number ?" .telephone number please I think only use fast food shop crew only. but now many young Japanese ask this way my beJapanese Mac Donald's infulence.