Is it true that Plus mobile operator will shut down its 3G network in about six months?

I use Samsung GT-S5670, Nokia 108 RM-944, and LG KP100 phones, and all my numbers are with Plus. Is it true that when Plus shuts down its 3G network, the phones will switch to 2G, or will they stop working entirely? If they switch to 2G, will the call quality be worse compared to 3G during phone conversations?