It's pretty straightforward config for a single WAN.
Disable SIP ALG on the Sonicwall ["SIP transforms"].
Forward the SIP and RTP ports to the phone system.
Make sure your ACLs only allow SIP from where SIP is really needed from.
Make sure public IP is set correctly in:
System\IP Settings\System NAT IP Address
System\Devices and Feature Codes\IP Connections\P6000\NAT IP Address
If you are using two WANs, however, you will have to use the Sonicwall ALG as you can't have more than public on the MiVO250.