Question:

Can one fulfill his obligation of reading the Megillah over the phone?

Discussion: 

There are really three variations of this question. The first is can one hear the Megillah over the phone. The second is can one listen to the Megillah through a microphone. The third is can one use a hearing aid and fulfill his obligation of reading the Megillah.

According to Rav Shlomo Zalman Auerbach zt"l one is does not fulfill his obligation if he hears the Megillah through a speaker system. This is because the sound one hears is not the actual voice, rather, an electronic replication of the person's voice (Minchas Shlomo 1:9). The same would be true if he heard it over a telephone. If a person is deaf or is hard of hearing and has a hearing aid, he should ask a rav what the proper course of action is.  

Rav Moshe zt"l agrees that one should not use a microphone or hear the Megillah over the phone. However, his reason is different then Rav Auerbach's reason (Iggros Moshe O.C. 2:108).

According to Rav Shmuel Kaminetsky, (Kovetz Halachos pg. 84) a person should take out his hearing aids to listen to the Megillah. The reason for this is since the main sound he hears is coming through the hearing aid he cannot fulfill his obligation while wearing them.           

Summary: 

One needs to hear the sound directly from the one reading the Megillah. Therefore, one cannot hear the Megillah over the phone or through a microphone. According to Rav Kaminetsky zt"l, a person with a hearing aid should take out his hearing aid and stand as close as he can to the chazzan. If one cannot hear at all without his hearing aid a rav should be consulted.