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
iLBC uses slightly more bandwidth (like 4kb/s) than G729 and in my opinion a better codec quality wise and handles packet loss better. Not worth paying extra for a service that licenses G729 vs a free service with iLBC.
I use switchtel dirt cheap with their billing and monthly rental, They have a good billing portal and you can view all details of call received and dialled. I don't have a G729 codec and quality is crystal clear even when calling from mobiles.
This is all setup on my asterisk home PBX. I have some family members using zoiper to register to asterisk, myself i use bria. No complaints