Complex sentence: The first telephones in the 1870s were rented in pairs which could only talk to each other, but the example of a central exchange was soon found to be even more advantageous than in telegraphy.
Split into simpler sentences: The first telephones in the 1870s were rented in pairs which were limited to conversation between those two instruments. The use of a central exchange was soon found to be even more advantageous than in telegraphy.