Please correct me if I am wrong.
You might see a increase in the Call drops when moving to AMR from EFR. This is not a problem with the AMR codec. What is happening is that the AMR codec is so much better that when a customer is using his phone on EFR and his call quality will go bad and the customer will end the call for example they are getting garbled voice. This will not peg as a call drop. But with AMR the call quality will stay better longer. The customer will keep talking on the call until the customer walks into a area in the building that has no coverage and the call will get dropped. Both will be frustrating to the customer. The one the customer disconnects there own call. The AMR will seem to hold the call longer but will peg a drop because the customer had good enough quality not to disconnect the call themselves.