Zoiper is a free SIP client that supports both SIP and IAX calls over 3G or WiFi connections. Zoiper has a simple user interface, with few menu items. Zoiper can work on most devices that are 2.1 and up. To begin you will need to have the SIP client installed, if you do not, you can download it for Android here, or you can download it for iOS devices 4.3 and higher here.
After opening the app you will be greeted with the Dialer screen.
Next go to the Config Tab and select Accounts
In the Accounts Tab you will have the option to add and manage Accounts. Select Add then SIP
To begin you will need your account information, then go to the Config tab, and select the Accounts then Add account
Account Name: IPComms this will be used as an identifier for the account
Username: NPANXXXXXX (this will be your SIP User)
Password: Password (this is your SIP password)
After testing this app on both 3G and WiFi networks, we were able to make and receive calls with no issues. Since the app integrates your phonebook, calls can be quickly made through the app to any of your contacts. This softphone does not contain any ads, which helps to not drain your battery.