What is the nature of the signal given by the needle of a turntable?
The needle or pickup is very sensitive and gives out a very low signal -  maybe 10 mV or so. Connecting the relatively high level headphone or "line-level" signal from your phone - typically 1 V or so - would cause great distortion. Turning down the phone doesn't really help as the background noise from the phone audio output would not get turned down (as much of this would be generated after the phone's volume control circuit).

Your best bet is to see if you can find the volume control on the record player and hook in there.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The modification.


Assuming it's a mono player it will only have one volume control. Your phone will be stereo so you need to feed both channels into the mono volume control.
The pickup will be pre-amplified and fed to the volume control before being fed to the "power" amplifier to drive the loudspeaker.
You need to find the clockwise end of the potentiometer and connect your audio in as shown in Figure 1. The 10k resistors "mix" the audio and limit the current into the pre-amplifier for safety.