Although it lacks a notification in WhatsApp it's just as easy to check a phone number for 'aliveness'. When you select 'new conversation' you get a list of you

Although it lacks a notification in WhatsApp it's just as easy to check a phone number for 'aliveness'. When you select 'new conversation' you get a list of your contacts with a name (from your address book) and a picture and tagline. This is all based on your address-book so it doesn't matter if the other party knows your number or not.