Bria iPhone Edition. 

Settings -> Accounts: 

Account Name: CWU 
Enabled: ON 
Incoming Calls: ON 
Display as: Your name
Username: xxxxxxxxx (your 9-digit username) 
Password: <your 6 digit SIP password>
Domain: sip.callwithus.com 

Account Advanced: 

Out. Proxy: [domain is the default] 
Auth Username: [username is the default] 
Global IP: OFF 
SIP Transport: udp 
Encrypt Audio: Never 
Interval: 120

Settings -> Advanced Settings: 

Strategy: Server Managed 
Enabled Audio Codecs: G729a (if bought), G711a, G711u.