Phono preamp picking EMI (4G/LTE/GSM ??)
The problem is operational amplifiers can rectify high frequency signals down to lower frequencies if they are getting into the sensor or wires connected to the amplifier. So even if you do have a nice enclosure, the signals can still get in if you have external wires coming into the box. This can happen two ways:



Source:  EMI Rejection Ratio of Operational Amplifiers 

Capacitive coupling is due to the electric field and can be modeled as capacitance between your circuit and the radiating circuit (such as your cellphone, although it isn't grounded the model will still work if you consider an additional cap between grounds) 

Inductive is from the magnetic field component and usually comes in through wires coupling to each other or stray magnetic fields.

So what can you do about it? If the signal to the preamp is external, then use shielded cables to block electric fields from reaching the signals you want to protect. Another thing to consider is to also have a box around the microphone also. An ideal shield will extend all the way around the sensor and preamp. 

If this is not possible then you need to block the signal, high frequency signals will take the path of lowest impedance. There are two ways to do this: 


Block the high frequency signal with more inductance, ferrite beads, common mode chokes 
Shunt the high frequency signal back to the source by providing a low impedance pathway, with a capacitor


Ferrite beads can go a long way in blocking high frequency signals  from reaching your electronics before the pre amp. (since we don't know about your preamp or box I can only comment generally due to your lack of specificity, this is as far as I can go, learn how to communicate) 

Another thing you could also use a common mode choke on the input to block signals that are common to the amplifier. 

If you have any wires, twisting them will help avoid loops that can pick up EMI.

If this is a box, you might want to consider filtering on the output also as the box and external wires can act like antennas.