Electronic – Is using Zener diode with a voltage divider sensible

Zener diodes are characterized for reverse-biased operation at a specific current, designated I_z. For the indicated family of Zeners, this current is 5 mA, as given in Table 8 of the Datasheet.

This means, in order to use the Zener diode as a voltage regulator of the specified nominal voltage, it must be passing the stated I_z current, plus or minus a milliampere or so.

Thus, for the BZV55-4V7, the resistance used must be calculated such that it allows approximately 5 mA at (V_cc - 4.7) Volts.

For example: if the supply voltage is 9 Volts, then required resistance R = V / I = 860 Ohms. The nearest standard (E12 series, i.e. 10% tolerance) resistor value is 820 Ohms, which will work fine for the purpose.

The proposed 10 kOhms, on the other hand, will not allow sufficient current to pass for the Zener effect to kick in for usable regulation.

1. A transistor is a different type of (active) component entirely, to a passive component such as a resistor/capacitor/inductor. They have many different characteristics listed in the datasheet, one of the most important being current gain. If you don't understand much about them I suggest you get hold of a decent introductory electronics book and read up about them. There are thousands of part numbers, but in many cases you could use many different options. In a circuit such as this most general purpose NPN transistors will do (2N2222, 2N3904, BC337. etc)

2. The zener appears to be used in order to ensure the voltage does not rise above 3.3V (the zener will start conducting above this voltage) thereby protecting the 3V phone input from overvoltage. You don't need to calculate this value, you simply need a 3.3V rated zener, of which there will be many part numbers to choose from (assuming this is what the circuit requires)

EDIT - regarding more detail on why the transistors can be swapped, it's hard to explain briefly. They do have similar specifications, and are available in the common and easy to use through hole TO-92 package. The circuit shown is not ideal, but for a rough shifting circuit it will probably work with some distortion (how well depends somewhat on the input signals DC bias level)
Since important parameters like current gain can vary widely with temperature and between parts of the same number, a good transistor circuit will not rely on these parameters but control them with some basic feedback (e.g. by using an emitter resistor in a common emitter circuit)
Any good electronics book will have a section on how to use a transistor, understand it's parameters and design a circuit that meets your specifications with them. The website All About Circuits is pretty good (see section 3 for semiconductors), and book wise this question and it's answers should be of use: Basic Electronics Book. With a few hours of reading and a couple of breadboard experiments you should begin to see things more clearly.