As /u/ripsmileyculture already mentioned, the phonology, morphology, syntax, etc. of a language are all fundamentally aspects of its grammar, and not necessarily clearly distinct from each other. A phonological change might have consequences on other areas of a language's grammar. The noun-case system of Classical Latin, for example, eventually collapsed from a system of five, to two, to zero cases as many of the morphemes that distinguished them became syncretic (phonemically identical) as a result of sound change. Nominative rosa, accusative rosam, and ablative rosā (a 1st-declension noun meaning 'rose') all merged into the single form rosa as later stages of Latin deleted word-final /-m/ and lost the distinction between short and long vowels /a/ and /aː/.

I do however think it's safe to say that, in theory, there's no reason why two extremely similar "dialects" couldn't be distinguishable based solely on different phonological properties. The varieties of English spoken in New England can famously be divided into four main dialects (Northeastern, Northwestern, Southeastern, and Southwestern) based on only two variable properties. The Northern dialects have merged the vowel phonemes /ɑː/ and /ɔ/, a completion of the cot-caught merger, to [ä~ɑ] in all environments (though [ɔ] stil occurs before syllable-final [-ɹ]). The southern varieties, on the other hand, have left them distinct. Similarly, the Western dialects (as in General American English) are rhotic, pronouncing /ɹ/ wherever it appears, as opposed to the Eastern varieties which (like most varieties spoken in England) are non-rhotic, and don't pronounce /ɹ/ except at the beginning of syllables (though [ɹ] also appears allophonically as an epenthetic "linking" or "intruding R", which is inserted between adjacent vowels, across word-boundaries). Based on these differences alone, the phrase "Law and Order" would be pronounced, broadly, as follows in the four New England dialects:

Northwestern: [ˈlɑː ænd ˈɔɹ.ɾɚ]

Southwestern: [ˈlɔ  ænd ˈɔɹ.ɾɚ]

Northeastern: [ˈläɹ‿ænd ˈäː.ɾə]

Southeastern: [ˈlɔɹ‿ænd ˈɔː.ɾə]

Of course, this is a gross oversimplification of New England English and there are other differences that occur between these dialects, as well as between the idiolects of speakers of each individual dialect.