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.
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.