I dont understand how you get increased erlang when AMR is activated. AMR is a codec method therefore improves speech quality espcially in cases where frequency reuse is tight and interference may cause a problem. Activating AMR does not give you more timeslots therefore your Erlang does not increase. However AMR does give the operator the option to increase capacity at a site with tighter frequecy reuse so I guess you could increase your erlang that way. There is no way by just enabling AMR that you increase your erlang by nearly 3 x the amount.