What is the most likely cause of interference to a non-cordless telephone from a nearby transmitter?

The most likely cause of interference to a non-cordless telephone from a nearby transmitter is due to the telephone inadvertently acting as a radio receiver. Non-cordless telephones are wired devices that can pick up radio frequency signals, especially if they are not properly filtered or shielded. When a nearby transmitter is operating, it can emit radio frequency energy that may be coupled into the telephone's wiring. The telephone's circuitry can inadvertently demodulate these signals, resulting in audible interference such as static or other noises during a call.

In this scenario, the design of the telephone might make it susceptible to receiving RF signals, particularly if the transmitter is emitting at a frequency that overlaps with the typical audio frequency range of the device. Unlike cordless phones, which operate on specific radio frequencies and are designed to filter out interference, non-cordless telephones lack such protections, making them more vulnerable to unwanted signals.

While harmonics from the transmitter, poor station grounding, and improper transmitter adjustments can indeed cause interference issues in different contexts, they are not as directly relevant to the way a wired telephone could pick up and demodulate radio signals. Thus, the telephone acting as a radio receiver is the most direct and likely explanation for the interference observed.