You can look at any of the SIP providers (Freshphone, Greydot mobile, MWeb Talk, Vox, Axxess) since all of them have support at least one of the free codecs (G711a/u, ILBC, Gsm, Speex) in zoiper. I have not tried any of the premium codecs like G729 which use less bandwidth.
In general, on network calls are free and you only pay for calls terminating on another network, i.e. when you phone telkom etc. The thing to watch out for would be voice call quality. I have made calls where people mentioned that the sound is not great, so it might be worth testing each provider to see what works for you.
You can contact BITCO who seem to have free privileges to the G729 codec which works well with MicroSIP which provides free use of G729 as well. drop me a message if you need more help around this and setting up devices etc