A phoneme is not actually a sound. That's what a phone is. Here's phonemics 101
Speech sounds are infinitely diverse. Every time you say "linguistics", the initial phone ("l") is a distinct event -- its duration is different, its pitch and volume change in different ways, etc. In short, if you recorded one million phones as the same person said "linguistics" over and over again, each recording would be different.
But the human mind typically will not deal directly with this infinite variety of phones. Instead, what you hear gets sorted through your phonemic grid to classify each phone as an instance of some phoneme.
Example: When a native of Havana, Cuba says the name of the place they're from, the "v" phone you hear is neither a typical English "v" nor a typical English "b". If you're using your English phonemic grid, however, you will hear that phone as a variant example of one of those phonemes.
Similarly, when a native speaker of American English says "t-shirt" and "teacher" to a native speaker of Cuban Spanish, the listener may hear essentially the same thing both times. One reason is that the "sh" and "ch" phones they hear will probably sort into the same phoneme based on the listener's phonemic grid.