I've also had this issue, you can try to add this "no tone dialtone remote-onhook" under your dial-peer to see if that fixes this. It did for me on various models of VGs with SCCP controlled FXS ports.
An example
dial-peer voice 214 pots
no tone dialtone remote-onhook
service stcapp
port 2/14
Sincerely
Roger