Shias state, very clearly, that only Allah has the attribute of All-Hearing.

This means:

• Allaah can hear from an unbounded (unlimited) number of incoming communication streams.
  
  Translation:  Allaah can hear from an unlimited number of people at once
  
  
• Allaah can hear our call, regardless of our distance to Him.
  
  Translation:  Allaah can hear everyone from everywhere

Shias agree with those two statements, and again, they will clearly state the ONLY Allaah has this attribute of All-Hearing.

Of course, anyone who claims to be Muslim should hold this belief, as Allaahu, 'azza wa jall, states:

لَيْسَ كَمِثْلِهِ شَيْءٌ وَهُوَ السَّمِيعُ البَصِيرُ

"There is nothing like unto Him, and He is the All-Hearer, the All-Seer."
     (Quran, Surah Ash-shura 42:11)

Only Allaah has the attribute of All-Hearer.  Unfortunately, although many claim to believe in and adhere to the oneness of this attribute (only for Allaah), when we examine said claims, we find grave inconsistencies.

Shias, admittedly, call upon Ali, Fatemah, Hussein, Hasan, and a whole slew of other figures.  Without delving into the permissibility of calling upon other than Allaah, we first want to understand the implication made when one calls upon any of the aforementioned individuals.  Below is a graphic demonstrating the DU'A that Shias make:

Do the following individuals have the attribute of All-Hearing?

Compare the above graphic to the one demonstrating Allaah's attribute of All-Hearing.  They are effectively the SAME picture!

In summary, those who adhere to the Shia faith tradition claim that only Allaah is All-Hearing, yet when said claim is examined, we find that the attribute of All-Hearing is applied to a whole herd of individuals.

At the same time, we give good assumptions and assume that these individuals do this out of tradition and ignorance.  As a previous Christian and one who, unfortunately, participated in seemingly identical actions (calling upon Jesus, Mary, saints, etc), I can fully sympathize with the challenge of breaking away from the core tenents that one was raised with.  However, continuing to give good assumptions, we hope that Shias, and all Muslims for that matter, seek the truth and want to worship Allaahu, subhaanahu wa ta'aala alone, and without partners.  So we make du'a to Allaahu, 'azza wa jall, asking Him to guide us and them, and we continue our respectful efforts at explaining the pure and simplistic monotheistic message of Islaam to all.