I have succesfully configured SIPTAPI by IPCOM for use with the Cisco UC520.
Follow the installation instructions and then configure like this.
SIP DOMAIN: IP for the UC520, this can either be the LAN IP or the CallManager IP.
SIP PROXY: leave this empty
user: leave this empty
password: leave this empty
extension: your extension number
If configured like this, whenever you dial from dialer.exe or outlook your own extension will ring, pick it up and the number you requested will be dialed.