Why is my verizon htc showing no network?

[AC Question]

Hellow there,l got my verizon htc phone from uk and am currently in kenya,but when l insert my kenyan safaricom line,its asks for the pin and accepts,l can also see the contacts on the simcard but no network so it cant call or be called,or you cant txt or receive a txt....it says the simcard is unknown or when you try to call it says,the call failured.can you help?