Two devices, one network outlet. That's were a splitter can help. I needed one for a computer and a phone, and decided to build it. The phone uses the ISDN protocol and the computer Ethernet. From my research, it looked like it is a rather unusual combination for a splitter. That comforted me into building my own pair.
Gigabit Ethernet over copper wiring (1000BASE-T) uses the 8 wires in the cable but if you only provide some 4 wires, it will usually downgrade to Fast Ethernet (typically 100BASE-TX) with a maximum bandwidth of 100 Mbps. That's a tradeoff I had to accept because there was no easy way to install a second cable in the wall. ISDN, on the other hand, only requires four wires, which was fine in this case.
If you want to build your own pair of splitters, I recommend you have a look at the Ethernet splitter guide on www.instructables.com. Just make sure you use the following Ethernet-ISDN wiring on both sides instead of the Ethernet-Ethernet one shown in the guide:
shared 1 2 3 4 5 6 7 8 / / / | | | \ \ / / / | \/ | | / / / | /\ | | / / / \/ \ | | / / / /\ \ | | / / / _/ \_ \|_| / / / / \ | |\ / / / / \ \ \ \ / / / x x / x x x x \ \ \ \ x x / / / / / / / / \ \ \ \ \ \ \ \ 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 Ethernet ISDN