Solved! How can I activate my SIM out of USA. Because I am in Cameroon and I want to insert a different SIM.

Solution
SIMs are activated by the carrier/service provider. Only they can activate it.

Now if the phone won't accept the SIM and won't let you use a different SIM then the phone may be locked to your old carrier/service provider. If that is the case, you need to contact them to unlock it. No one else can unlock it.
SIMs are activated by the carrier/service provider. Only they can activate it.

Now if the phone won't accept the SIM and won't let you use a different SIM then the phone may be locked to your old carrier/service provider. If that is the case, you need to contact them to unlock it. No one else can unlock it.
 
Solution