Ex Speedbird FYI
If it wasn't for your explanation about being very careful dialling (as we still call it even tho dials are long gone) I would have suggested finger trouble as of course 4 and 7 are adjacent.
On the technicalities the traditional (but still rotatory dial) phone network used what was called Dual Tone Multi frequency signalling (DTMF) which transmitted the numbers by using a code made up of two frequencies out of five - you could often here this 'tune ' if you dialled a number and then pressed redial so they were sent quickly one after the other. People in the US like the blue box man used to covet phone numbers that paled the opening bars to Dixie of the national anthem.
But back to your problem. If you are calling computer to computer these tones are not used the number starts out in the standard keyboard code for numerics and is sent as a digital signal all the way to the far end .
If you are using one of the systems where the internet carries the call say from NZ to the UK but to an ordinary phone number (say like Skype out , where the called party) doesn't have a PC) then there is room for error as the internet digital code has to be converted back in to phone network speak and there is the possibility of a fault there.
If you are phoning from an ordinary phone there is again the possibility that the wrong frequency pair is sent for the wrong digit but that shouldn't affect you on voip.
So your options are
1 You have a voip phone and the 4 or 7 key makes a bad contact sometimes -they are adjacent
2 You use a Skype out type service where the far end is Offnet and the conversion back to ph0ne signalling is faulty
3 You are using end to end on net and the problem is your pc keyboard if you are using laptop or desktop-try typing a string of fours and see if the occasional seven appears.
Not much help I am afraid but they are the fault options.
As to the calculator keypad difference that was a big issue back in the 70s when a huge amount of effort went into discussing the layout for a phone keypad -including the names and symbols for what became * and #.
The reason they are not the same is really one for the calculator people since in the USA DTMF -push button -phones were there before calculators .
And finally about 999 it is easy to find but on the old rotary phones it used to take the dial three seconds to send the signal whereas 911 on a US dial phone took only 1.3 seconds because the ones take less time that the 9s to send the info to the exchange. That's whey New York is area code 212 because 1 and 9 were never used as the first digit of phone numbers and as New York received the most calls it got the code that dial fastest. And yes someone did work that out for all of America because it actually made difference to the giant clockwork electromechanical switches how fast the numbers were sent, hence NY 212 LA 213 Chicago 312 and so on
PB