On WBC the ISP has to pick up their user traffic at 20 nodes scattered around the country, and the node yours goes to depends on your exchange, and in a few cases even the MSAN within the exchange.
That makes calculating the necessary capacity to rent from BT at each node rather difficult for the ISP, so congestion can occur there.
On WBMC normally, (perhaps always), the ISP picks up the traffic at a single node. That is easier for them to manage as they have a reasonable idea of their overall peak demand, but I assume there are other drawbacks.
Which is why I raised the question in my previous post, as I don't know which method Zen use.
There could of course be simple busy-time congestion on the BT side of things at your exchange.
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