The problem with G.723.1 is that terminating partners in Africa generally don't support it, so you're going to have transcoding happening at the carrier and quality degrades hugely with transcoding. You may find that where the route into Africa is a TDM interconnect (clear G.711 A-law channel) it works brilliantly, but for other countries where the route transcodes to G.729, it works terribly.
Thankfully, G.729 is almost universally supported natively, and a G.729 to/from G.711 A-law transcode will achieve a good MoS.
In reality, how often are all 12 phones going to be in used on live calls at the same time? Keep in mind that often people are busy dialling, ringing, getting engaged tones, hanging up, holding up a line without actually generating call traffic. This is particularly so in a call shop where people are in and out, paying, etc. It is quite likely that you may only peak at 9 or so live active calls at once.
Also, good luck getting the full 512kb/s to/from an international VoIP provider. I see there have been recommendations about bandwidth providers and, yes, Verizon / SAIX unshaped will give better results, but it is still extremely optimistic expecting such a consistent high-bandwidth throughput to/from an international provider.
If you are peaking with 12 calls (e.g. If you have 16+ phones), then you really shouldn't be stuffing around with this cheap nasty solution, but rather setting up a Trixbox/Asterisk PBX with IAX2 trunking to a local ITSP. This will let you easily carry 30+ (even up to 50+) simultaneous G.729 calls.
The problem is going to be finding an obliging local ITSP who can give you the rates you want and the IAX2 termination. You need to calculate the cost of multiple ADSL lines, unshaped vs local internet bandwidth, etc, vs the per-minute call costs. It may be cheaper paying higher rates, or it may be cheaper getting more expensive bandwidth and using two ADSLs...
