Acrobits Groundwire and Acrobits Softphone. 

Title: CWU 
Username: xxxxxxxxx (your 9-digit username) 
Password: <your SIP password>
Domain: sip.callwithus.com 
Display Name: Your name
Incoming Calls: On 

Advanced settings:  

Auth User Name: xxxxxxxxx (your 9-digit username) 
Transport Protocol: tcp
Proxy: leave empty  
Proxy Port: 5060 
STUN Server: stun.callwithus.com 
Discover Global IP: off (unless you have some issues with incoming calls) 
Codecs For WiFi: G.729a (if bought), G.711 u-Law, G.711 a-Law. 
Codecs For 3G: G.729a (if bought), G.711 u-Law, G.711 a-Law. 
G.729a is highly recommended for 3G calls! 
RTP Port Start: 16384 
RTP Port End: 16482 
Expires: 120