You can use IP-Phone-C and NTT's Hikari Denwa at the same time
However, you will need to change the wiring, and there are restrictions in use. Please refer to the following page for details.

NTT EAST: Precautions for the Use of Hikari Denwa(Using together with 050 IP-Phone) (Japanese)

NTT WEST: Precautions for the Use of Hikari Denwa(Using together with 050 IP-Phone) (Japanese)