It is necessary to listen during a class; however, retaining information is just as imperative. Entertaining more than one conversation at a time will focus your listening skills to more than one source, hence limiting what you will retain. Asking your classmate about last night's game may cost you a quarter of the multiple choice section of your biology final.
No one likes a chatterbox, especially not professors. Most educators have spent years of studying and research to reach the status of professor and demand a certain level of respect during their classes. Unlike the occasional side conversation, constant texting and talking are considered rude and will leave a bad impression on your professor.
Even if you are not the one participating in a side conversation, if one is going on around you, this may affect how well you concentrate. Phone conversations are especially a problem because you are hearing only half of a conversation, so your brain signals get confused and strain to make sense of what is going on. This action will distract you from what is going on during the lesson and break your concentration.
Engaging other conversations during class time will ultimately affect your overall performance. Your level of commitment is jeopardized. Your participation grade may be affected because of your class disruptions. Your test scores may be lower than your potential because you did not put forth 100 percent focus during the lecture to grasp all of the concepts.