Shm- reduplication is a linguistic habit that English has picked up from Yiddish, in which one reduplicates a word, but replaces the initial consonant or consonant cluster of the second production with 'shm'. The most common example is probably fancy-shmancy (more commonly spelled fancy-schmancy)... although that is actually a bad example.
Most often, when shm- reduplication is used it indicates disregard, disagreement, sarcasm, or disrespect. 'Phone-shmone' indicates that one in not in favor of using the phone right now, or doesn't like this specific phone.
There are at least two constructions in which this is not the case. First, a fancy-schmancy restaurant is likely to actually be quite fancy; you will have to rely on tone and context to determine if the reduplication is acting as a negative or an intensifier. Second, Joe Shmoe is used in much the same way as John Doe is, albeit with a negative connotation indicating that this person is no one important.
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