English, like some other languages (e.g., Russian), has so-called simplification rules whereby a consonant (usually medial) will drop out of a cluster in pronunciation. Thus words like glisten, hasten, whistle, trestle, etc. are pronounced without the [t] before /n/ and /l/. A cluster like /-rtg-/ in mortgage drops the [t] as well. In fact the sound /t/ in medial position in a cluster of three consonants typically syncopates (drops out) whatever consonants surround it. Other consonants also may syncopate episodically, viz. the [b] in clamber, although in this case a (non-traditional) spelling pronunciation is also extant, whereby the [b] is retained (cf. limber).
While the common explanation of such cases of syncope has resorted to phonetic factors such as the notorious “economy/ease of effort,” a systematic phonological purview makes it clear that what is at stake is the semeiotic relation between the supervenient phonological (markedness) values of the sounds involved and the rules of combination (the phonetic pattern) determining pronunciation. Specifically, the rules of combination are an ICON of the phonological values. In the particular case of consonant syncope, what the rules map are the fact that the consonant syncopated is MULTIPLY MARKED for one relevant feature or another. Thus /t/ is marked for both the features grave vs. acute and strident vs. mellow, so the fact that it drops out from the relevant consonantal cluster is to be properly regarded as an iconic realization of its definition in the phonological structure of English.