said by atcotr :You can't "split" Ethernet connections like you can with analog phone lines. Each PC, phone, or gadget must have its own line back to a switch.
I imagine he is using the wrong terminology as you are quite correct.
@OP please give us a part number or a model number. Anything more descriptive of both the "splitter" and the Voip phone. I suspect you may have a switch and the VOIP phone is not getting assigned an IP address. It may be necessary to assign everything manually. DHCP nay not work behind the switch.