imessage problem

working with iphone 3gs and ios5 i activated icloud and usage of imessage. it works - i can send imessages from my 3gs to people ´s phonenumber.

i have my apple id and phone number registered and the phone number is active in messages.

if i send an imessage to a phonenumber the answer arrives as imessage.

but if someone wants to imessage me that person can only mail me a message (so a sms).

why can other ios5 and iphone users with my adress and handynumber in their adressbook not communicate with me using imessage ?

if they start a communication they wont see imessage - they just see message using the phonenumber.

i did all resets, deregistered and registered back but still no advance. also i enabled and disabled restrictions and what i found in forums.

before calling apple, can somebody help ? a friend of mine now has the same problem with his girl friend´s iphone 4. so the reason of this problem cannot be my older 3gs.

addressbook mistake ? any solutions ?

appreciate

cj