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There are 2 different ways to do so depending on the device you own. On some HSPA/LTE devices you have to dial *61* your cell phone number + time value (1 ring = 5seconds) + #+ send. On other HSPA/LTE devices You have to dial *004*+your number+time value (1 ring= 5 seconds) + # + send. To deactivate it you either have to dial #61# + send or #004# + send.