In these logs I will show how to retrofit your vintage rotary phone. I'll try to describe the main steps to modify the phone.

The first step is to put together the DF mini player, the Wemos board and the old handset.

The DFplayer mini is an MP3 player that can talk with your preferred microcontroller through serial communication with two wires.

In this log we connect the player with the Wemos board. There is just a thing to pay attention to before connecting. The DFplayer has a 3V logic, and Wemos has a 5V logic. This means that a High signal is transmitted with 5V from Wemos and thus will damage your DF player. 

So we need to put a resistor between the receiving pin of DF player and the transmitting pin of Wemos, in this kind of two pins wired communication the RX pin of a board is connected to the TX of the other device, and viceversa.

In this case the second pin it's not necessary since the DF player is transmitting and Wemos is receiving and the 3V high signal is correctly received as an High signal also from Wemos, because it's more then half of 5V. 

Everything above 2.5V is considered high, and everything under 2.5V is considered low. So it works.

The DF player has two pins who bring left and right audio signals to the speaker and we connect them with cables coming out of the handset.

To connect the handset you have to find the two proper cables out of the three cables available. 

TIP: When you open the old phone, disconnect all the connectors from the parts you want to use, you need to access the cables of the handset and no other wiring should be linked to the rest of the old phone or this will change the behaviour of the system.

Since I don't want to open the receiver, I've just tried two of the three cables. In my handset the cables to use are red and blue.

Here is the schema:

You can upload a few mp3 on your micro SD card, insert the cart in the player and test the process with the DF mini example scripts of the Arduino IDE library DFminiMp3. 

I've used version 1.07.

When you try the PlayMp3.ino example sketch you have to modify some lines, since in the Wemos board we use a Serial Software and not an Hardware software:

So comment line 65 and uncomment 69 and 70.

At line 69 change 10 with D6 and 11 with D5.

If you hear your mp3 in the handset you did it!

If not check these things:

• check pin connections RX-TX and resistor on pin D5 as described in the above schema.
• check folders name in your SD card, you should have only a folder named mp3 and inside a few mp3 files with names that are numbers zeropadded: 0000.mp3, 0001.mp3... and so on. Four digits numbered files.
• if none of above, think about adding an external power source