30 Ohms seems a bit unusual. Most mainline sounders are 100 to 150 ohms. Local sounders are usually 4 ohms, with a relay in the mainline and the relay contacts driving the 4 ohm sounder via a local battery (around 1.5V).
I'm going to go out on a limb and suggest that 30 ohms might be used for a large building intercom wire. Longer loop than just the three feet needed to connect a 4 ohm sounder to relay contacts and a local battery, but probably not more than 2000 ft of loop wire connecting two to four sounders within a big building. I've been told that for 100 mile loops using open wire along a RR right of way it was common to have 48 or 130V DC sources at each end of the wire (positive at one end, negative at the other, incandescent bulbs in series at each end to limit current if the line shorts to ground) and use earth ground to complete the loop. Lots of good wireline telegraph info in the "AT&T Green Book" (PDFs available on the internet).
But you can find the people who really know their stuff about landline telegraphy at
MorseLandlineTelegraphy@groups.io rather than relying on my fourth hand information.
73, Paul, ad7i
Middletown, NJ