Is it true that the congregant should respond to the muezzin's call for the funeral prayer by saying, "O Allah, forgive us and him"?

The listener to the call for prayer over the deceased is not obligated to answer the caller with a supplication. However, if he supplicates for the deceased for forgiveness, there is no harm in it. Supplicating for the dead is desirable, but there is no evidence for answering the caller with a supplication.