The following configuration will enable transcoding on a Cisco Call Manager Express Voice Gateway.

Note: CME is listening on interface Vlan 48 with IP address 10.14.48.1

voice-card 0
 dsp services dspfarm
!
sccp local Vlan48
sccp ccm 10.14.48.1 identifier 1
sccp
!
dspfarm profile 1 transcode
 codec g711ulaw
 codec g711alaw
 codec g729ar8
 codec g729abr8
 codec g729r8
 maximum sessions 4
 associate application SCCP
!
sccp ccm group 1
 associate ccm 1 priority 1
 associate profile 1 register MTP123456782012
 keepalive retries 5
 switchover method immediate
 switchback method immediate
 switchback interval 15
!
telephony-service
 sdspfarm units 1
 sdspfarm transcode sessions 4
 sdspfarm tag 1 MTP123456782012
 ip source-address 10.14.48.1 port 2000

References

Universal Voice Transcoding Support for IP-to-IP Gateways
http://cisco.biz/en/US/docs/ios/12_4t/12_4t15/it_unitr.html

Unified Border Element (CUBE) with Cisco Unified Communications Manager (CUCM) Configuration Example
http://www.cisco.com/en/US/products/sw/voicesw/ps5640/products_configuration_example09186a00808ead0f.shtml#transcoding

Configuring Transcoding Resources
http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmetrnsc.html

SIP-to-SIP Connections on a Cisco Unified Border Element
http://www.ciscosystems.com/en/US/docs/ios/voice/cube/configuration/guide/vb-gw-sipsip.html