What device do you currently have on your line? What device are you trying to activate?
If they use the same size SIM card, you can simply turn off your current device, take out the SIM card, put that card in the other phone and turn it on. You have just switched devices. I do it all the time. For phone which use the same size SIM card, there is no need to go online or call CS to make the switch. Simply move the SIM from one phone to the other.