Does anyone know if this is a Ethernet jack or a phone line?

Looks like an RJ25 for 3 phone lines. I used to have "business" phones that used these. Some people refer to them as RJ11, because RJ11 (1 pair for phone, sometimes an additional pair for power to the light in the phone), RJ14 (2 pair), and RJ25 (3 pair) all use the same 6 position (6P) connector size. RJ45 and the Networking equivalent are 8P8C. Old Category 5 10Mbps Ethernet and some 100Mbps "Fast" Ethernet can use 2 pairs but use pins 1,2,3,6 avoiding the center pair because back in 1990 you might have the phone and Ethernet on the same 4 pair cable. You can see more details at https://en.wikipedia.org/wiki/Registered_jack#RJ11,_RJ14,_RJ25_wiring and https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair.