1) Download and install Linphone (free ):
2) Go to the Iphone/Ipad Settings, choose Linphone
3) Fill in your sip account credentials and choose an audio codec

Sip account
User name = your sip account (e.g. 81287)
Password = your sip account password
Domain  = voiptigersip.com

Audio Codecs
Select your audio quality according to your available internet bandwidth.
You can choose between:
•    ILBC (low bandwidth, good audio quality)
•    PCMA (high bandwidth, very good audio quality)    
•    722 PCMA (high bandwidth, High definition audio quality)    

Leave all other settings as they are.
You are now ready to make (and receive) calls! Please make sure you have enough credit on your account.