This question comes up a lot, and it's a little complex, but something definitely worth understanding.
IPC is a networking component which allows DSL traffic to break out of Telkom's DSL network and onto an ISP's private network (in our case, MTN's national fibre network). The IPC's are housed at data centres, so that they break out directly onto our network. We have three regional IPC's, in Johannesburg (North region), Cape Town (South) and Durban (East). Telkom owns the IPC hardware and everything leading up to the IPC, so they are the only ones allowed to do upgrades or maintenance to the IPC (and essentially ISP must rent the ISP from Telkom, kind of like you rent your Telkom line from them, even though it lives on your property).
Each regional IPC represents the closest point that a user can break out of Telkom's network, so it's important for an ISP to have sufficient IPC capacity in order to allow enough traffic at any given time for the number of clients they have or the amount of traffic - specific to each region. When ISP's refer to upgrading capacity, they are often referring to IPC upgrades, which are performed exclusively by Telkom. In our case, DSL traffic goes from the IPC directly onto our national fibre network.
There are probably more technical explanations (hopefully someone will do the honours here) but this is about the simplest way to understand how it all works :)