It's not connected to anything, so probably not. However back when we used purpose-built electromechanical switching systems for telephones, people exploited them[0].

[0]https://en.m.wikipedia.org/wiki/Phreaking

If the phone system used water instead of electricity for in-band signaling, would phone phreaks have to inject blue fluid into the pipes to exploit them?

If water pipes were used for signalling, it would probably be as acoustic conduits (analogous to wires, which are electromagnetic wave conduits). You would exploit them in just about the same way as the electromechanical systems were.

Gives a whole new meaning to the term "wiretap".