Change on your config on osmo-bsc, osmo-msc to change the codec.

example :

(osmo-bsc)

msc 0
 ip.access rtp-base 25000
 no bsc-welcome-text
 no bsc-msc-lost-text
 no bsc-grace-text
 type normal
 allow-emergency allow
 codec-list fr1
 amr-config 12_2k forbidden
 amr-config 10_2k forbidden
 amr-config 7_95k forbidden
 amr-config 7_40k forbidden
 amr-config 6_70k forbidden
 amr-config 5_90k forbidden
 amr-config 5_15k forbidden
 amr-config 4_75k forbidden
 mgw remote-ip 127.0.0.1
 mgw remote-port 2427
 mgw endpoint-range 1 31
 mgw local-ip 127.0.0.1

(osmo-msc)

mncc-int
 default-codec tch-f fr
 default-codec tch-h hr