Official Explanation
Read the Original Sentence Carefully, Looking for Errors
The underlined segment begins with the verb phrase "is considered." Check that the verb agrees with the subject: the subject of this sentence is "number," so the singular "is" is correct. Remember that the subject of a clause will never be inside a prepositional phrase, so don't be distracted by the nouns in "of phone calls" and "from constituents. It's the number of calls that is unusual.
Also, the verb consider is part of a commonly tested idiom; the correct phrasing is X is considered Y (not considered to be). This sentence handles the idiom correctly with "The high number ... is considered unusual."
Another idiomatic usage issue could arise with "given," but again this word is used correctly here. The word given is followed directly by a noun or noun phrase. So, for example, it would be correct to say, Her grade was surprisingly high given the reputation of the teacher. That is the pattern followed here.
This sentence appears to contain no errors, so suspect (A) is the answer. Check the other choices to make sure you haven't overlooked an error and to see whether another choice expresses the same thought in a more concise way.
Scan and Group the Answer Choices
The 3-2 split between the singular verb phrase "is considered" and the plural verb phrase "are considered" stands out. The subject of the verb is the collective noun "number," which acts as a singular noun, so a singular verb is necessary. Eliminate (D) and (E).
Another 3-2 grouping concerns "considered." (B) and (E) use the incorrect "considered to be," while the other choices use "considered."
Eliminate Choices Until Only One Remains
Eliminate (D) and (E) for starting with a plural verb; (E) also commits a usage error as noted above. Get rid of (B) as well for its mishandling of "considered."
Now take a look at (C), which changes the idiomatically correct transition word "given" to the incorrect "with." The word "given" conveys that what follows provides an explanation as to why the high number of phone calls was unusual. The word "with" implies that the low turnout at the town meeting was also considered unusual. Having eliminated all other answer choices, you can be confident that (A) is the correct answer. This sentence is correct as written.
TAKEAWAY: Idioms do not follow an overarching set of rules, so take time to familiarize yourself with the idioms commonly tested on the GMAT.