Its a common issue with asterisk as it sometimes wont pass dtmf properly. To solve the issue first we need to check the network has sufficient bandwidth, if bandwidth is sufficient then we need to add the below parameters on all CV1 trunks. Note that if you are using G729 codec then inband wont work. First try with dtmfmode=auto, if it didn’t work then try with dtmfmode=inband.(For only ULAW) codec. If you are using g729, then force rfc2833 instead of inband.
dtmfmode=auto or (use inband for ULAW / Use rfc2833 for G729)