Why do cell phones sometimes interfere with amplifiers
The usual problem with interference from GSM cell phones is caused by rectification of the strong RF signal at the semiconductor junctions at the input of the amplifier.
GSM uses Time Domain Multiplexing to share the RF frequency with other users - it only turns the RF signal on for a short time at a 217Hz rate with other users and the base station using the remaining time.
This short relatively high-power RF can be rectified by any semiconductor junctions to cause a slight change in the bias level of the amplifier for the duration of the RF pulse (~0.5ms).  This is a buzz at 217Hz that can then get amplified by the amplifier to appear at the speakers.
It is not necessary for the amplifier to have any response at the RF frequency which can be up to about 2GHz.
The cure is to provide filtering to avoid the RF signal getting to the first stage of the amplifier.  The filtering may be as simple as a small high-frequency capacitor across the inputs, an RF choke in series or a more complex filter.
Bipolar junction transistors are more sensitive to this effect than FET front ends so it will be affected by which devices are used.
It's not obvious why SMS and data transfers would not cause noise as well - they use the same mechanism for transferring data.