What company can this phone be activated

You didn't list what phone it was. However, the device can only be connected to the original carrier/service provider you got it through until it is unlocked. Then it can be used on other networks. That said, it will only work on networks of the same time. Either CDMA or GSM. So knowing which is important. Your best bet is to get the original carrier/service provider to unlock it for you and then contact the carrier/service provider you want to change to and see if that phone will work with their network.