Yes but they mean different things. "Many phones ARE...." is saying that phones ARE made in China and are still continuing to be made in China. (It's still happening.) "Many phones WERE..." is past tense. Phones are not being made there anymore. OR Not as many phones are being made there now. Something has happened that has changed the amount of phones being made. example: "Many phones in the world WERE made in China BUT NOW many phones ARE made in other countries."

Both are grammatically correct. However, "were made" and "is made" deliver a different meaning. "Many phones in the world were made in China" indicates that the making of the phones are done, that it wont be made again. It happens in the past. On the contrary, "many phones in the world are made in China" delivers a general fact, that the phones are still in the making. I think it would be more correct if you use the first one ("are made").