Connect 2 Asterisk Boxes - Codec Problem


I want to connect two asterisk boxes via SIP connection. I have followed the instructions given in the book “Asterisk, The Future of Telephony” and succeeded in doing so.

The codec been used in between the two boxes is ulaw. Now I want to change it to gsm. When I tried to do so, I get the following error on dialing the extension

Any suggestions?