Before the conspiracy threads start I would like it noting that I am a telecoms engineer and if you can't get through to a line it's NOT a conspiracy, it's congestion!
Each number has a maximum load which is NOT balanced - ie, you'll struggle to get through tot the hot favourite but the rank outsider voter will have no problems.
This does in effect mean that BT/ITV won't register as many votes as if the maximum load had been higher (an almost physical impossibility with the surge of callers.) However, the person with the most attempted votes will always have the most actual votes too as every time a channel becomes free to accept the vote it will (almost per-second if people are calling non-stop). Whereas with an outsider there will be a wait period between votes, despite the 'success rate' of callers being 100%.
It's somewhat hard to explain but as long as the lines have an equal max load (which I'm sure they do, given ITVs past with OFCOM!) the person with the most votes will have gotten the max any of the lines could handle in that hour - so even if 10% of callers failed to get through, they would still win - because the callers only failed to get through because the channel was in-use for a successful vote.