asterisk users - Jan 2010 - Question about PLC of Asterisk

Hi,I want to know how to do to work PLC of Asterisk.
Anyone plz help me.

PLC (Packet Loss Concealment) is included in Asterisk,I read at voip-info.org or
release note.
And I see in codecs.conf, "genelicplc" setting.

So I put codecs.conf in '/etc/asterisk' ,and wrote "genericplc
=> true".
And I worked Asterisk and tested.

I think PLC like this. 
When I send 3 packet (1,2,3), and caused loss (if No.2 is lossed),
then receive-packet after PLC should be same number 3 packet(1,2,3).
No.2 is new one. I don't know 2 is same sound to old 2,or is zero level
sound.
But same number packets should be resend from Asterisk to receiver.

But when I send 3 packet, received-packet is 2 packet(1,3).
No.2 has lossed.
No interpolation has been done.

So I can't understand how to do to work PLC of Asterisk.

Please help me.

I tested Asterisk 1.4.* and 1.6.0.
Use uLaw to send and receive on SIP/RTP.
And caused 5-20% loss from send-packet and through Asterisk ,I checked
receive-packet.
 
Is there another things to do without "to put codecs.conf in
/etc/asterisk" ?
Or Asterisk has no PLC on uLaw ?