Pretty much all VoIP providers are the same.This could well be the problem. People in charge at work aren't as tech knowledgeable as they like to think and I have no doubt remote staff are jostling with VoIP for bandwidth.
Another thing is although WFH staff all have fibre at home, work is still on ADSL. Trying to catch up on revenue before paying exorbitant amount quoted by fibre providers to lay fibre from the street up the 40 metre driveway into the building.
Would the VoIP providers know which codec to use?
Depending on who your voip provider is, I would suggest getting another naked dsl line, no need for the phone line portion from telkom in this case.
If you use Vox for VoIP get a line through Vox, if you use Mweb for VoIP get a line through Mweb.
And configure all your sip phones to go through that dsl line, and no other traffic.
By using a dsl line from the same provider as the VoIP provider, you will avoid peering further up the line, thus avoiding any peering issues between ISPs, as the VoIP servers usually sit on the same network.
Generally when you have the same VoIP provider supply VoIP and internet, latency and jitter will be lower to their SIP servers, again no peering, and most will prioritise VoIP traffic over their network to their SIP servers.
Edit: to answer your question, 99% of the time G729 codec is used.
You would need a 10mbps ADSL to get 1mbps upload speed, which should handle about 20 odd concurrent calls using G729.
And about 8 concurrent calls using G711 (Alaw/Ulaw).