These setup instructions were made for the 7960. Other Cisco desktop models may be similar in setup.

Phone must have SIP firmware installed. To check, make sure the phone is powered up and connected to the network. Press the settings button, then status, then select, then firmware. The application load ID file must start with POS. The POS in the filename means that the phone has a SIP firmware version installed.

Notes from Cisco: http://www.cisco.com/pcgi-bin/tablebuild.pl/sip-ip-phone7960

Setup phone for MidwestSIP

Configure phone:
Press settings, select unlock config, enter password (default is cisco), select SIP configuration, select line 1 settings

Name: (MidwestSIP username) 
Shortname: (MidwestSIP username)
Authentication name: (MidwestSIP username)
Authentication Password: (MidwestSIP password)
Display Name: (MidwestSIP username)
Proxy Address: (sip.midwestsip.com)
Proxy Port: leave default as (5060)
Save
Go back to SIP Configuration, Change setting for Register with Proxy to (YES)
Go to Outbound Proxy, set to (sip.midwestsip.com) 

Phone should register now and be able to make inbound and outbound calls.

Please see you network administrator if the phone still does not dial as expected.