When your application raises a ACTION_CALL intent, the IntentResolver component of the runtime will build a list of all the applications that can cater to this Intent.
If there are more than one client who can serve as a provider, generally a user is displayed a chooser activity from which he/she can choose.
As far as formatting of the telephone number goes, AFAIK, all the numbers defined as per
IETF's RFC 3966 are considered valid.