Are you in the process of joining 48? What do you mean by "activate your phone"? Do you mean activate your SIM? How long are you waiting? If more than an hour or two then you probably need to contact 48 support. See the link in my signature below. If the problem is something else then you need to post some more detail to explain it.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.