Yeah, the phenomenon is known as "vowel devoicing," and there are some rules about when it happens... the "su" in "sumimasen" is pronounced because it's followed by "mi," which starts with a voiced consonant. It's not really pronounced in "faasuto" because both "su" and "to" start with unvoiced consonants. Wikipedia has an overview and Google has more details if you're interested.
Yeah, the phenomenon is known as "vowel devoicing," and there are some rules about when it happens... the "su" in "sumimasen" is pronounced because it's followed by "mi," which starts with a voiced consonant. It's not really pronounced in "faasuto" because both "su" and "to" start with unvoiced consonants. Wikipedia has an overview and Google has more details if you're interested.