Once the SIP number is mapped to your account as shown below, use your credentials to register it on your softphone. 

Which can be Zoiper, X-Lite e.t.c

For Zoiper you also have to change a few settings.
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And you're all set.
Here's a tutorial on how to make an outbound call.
​help.africastalking.com/voice/how-do-i-make-an-outbound-call-from-my-soft-phone

If none of the above work check out this article on why calls get aborted.

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