Two readings, two different words. This is yù, the appealing one, the character in 呼吁, and it is written 籲 in traditional Chinese. Read xū, the same simplified character means to sigh, as in 长吁短叹.
usage
Formal, and almost never standalone. It appears in 呼吁 (to call on), 吁请 and 吁求, the language of statements, petitions and news reports rather than conversation.
Left mouth radical, the indexing element. 吁 began as a cry of pleading, so the mouth anchor is literal: a sound going out. It sits with other voice-noises like 呼 (call out), 喊 (shout) and 叹 (sigh).
