Sadly it sounds like you've been given false info. All DSL is copper to the DSLAM and fibre backhaul from the DSLAMs onwards, using mostly Telkom's ATM network atop SDH, but in some areas using Metro Ethernet backhaul (mostly parts of JHB and PTA). The DSLAM for a particular area will usually be at the nearest Telkom exchange. In some instances, Telkom does roll out DSLAMs in street boxes, but this is rare. In either case, the same DSLAMs serve each area, whether you opt for 384kb/s, 512kb/s or 4mb/s service.
There isn't really a 1mbps or 4mbps product. They are both referred to by Telkom as "ADSL Fastest" and described as "up to 4mb/s." This means that they will sync the line at the highest RELIABLE rate up to 4Mb/s. It can be configured as 1Mb/s or 2Mb/s if the line keeps dropping at 4Mb/s, but it's the same service at the same price and you'll get the best that the copper is capable of up to the stated maximum.
From a Telkom perspective, the cost difference between 512kb/s ("DSL Faster") and up to 4Mb/s ("DSL Fastest") is R87 per month (ref:
http://www.telkom.co.za/products_services/dsl/cost_dsl_cost.html ).
That being said, if they're quoting a bundled price for the ADSL line and bandwidth, they may be selling the bandwidth to you cheaper to make up the R52 difference. (Assuming they're not just misguided or misleading you.)
But either way, I am extremely skeptical of what they've told you...
I'm not sure of your call volumes. 512kb/s ADSL has a 256kb/s upload. Assuming SIP/G.729, that's good for at most 4 to 5 simultaneous calls. If your Mitel is using G.711, you'll get only 2 simultaneous calls. If you exceed those limits, your calls will suddenly break up (and/or possibly drop). So perhaps also check the codecs in use. Keep in mind that if you have multiple codecs enabled, it may not be your first choice that is in use, as the codec will be negotiated with the VoIP provider's equipment.
If I were you, I'd try and approach this more scientifically. Once you've checked codecs, etc, if you suspect the ADSL, log into the modem and see how long it has been sync'ed for. Minutes? Hours? Days? Try and check the usage. Perhaps Google for a tool that will monitor the modem's line utilitsation using SNMP so you can see if it is running busy when you experience issues.
And then there is the simplest of all options: try get a trial voip service from an alternate VoIP provider. I am pretty sure that Switch Telecom (
www.switchtel.co.za) will assist you... as they are unofficially doing right now... ;-)