You have a voice gateway such as VG204, from CUCM 8.6 Device > Phone, you saw the newly configured VG204 Device Name (Port) status was rejected.
Check on your VG204 to see if there is dial-peer for the port that is rejected by CUCM.
show run | sec dial-peer
To configure the dial-peer, supposed the rejected port is port 0/1 of VG204.
dial-peer voice 10 pots