Which VOIP protocol are you using?
If using MGCP check:
debug mgcp packet and look for NOTIFY messages. Check
mgcp dtmf-relay voip codec all mode out-of-band command is configured.
If H225 check dtmf-relay alphanumeric under VOIP dial-peers.
For checking debugs run debug h245 asn1
For TDM run:
debug voip vtsp tone