I could be wrong but I'm pretty sure that two in Japanese is "ni". And seven is "nana" which is pretty close
Apparently, the numbers come from some kind of Japanese word play. Here’s a source I trust for character and ship days/numbers. The OP tries very hard to be accurate and she (I think she’s a she!) was fact-checked by a Japanese speaker.
I’m female. ;)
The readings mentioned by anon are the standard readings for Japanese numbers (see the “On reading” column here), while goroawase, or Japanese word play, makes use of a variety of different readings (some not very common) for the same number. You can see some of the readings used in goroawase here and here (second one is a pdf). As mentioned by imshiroi, there are no fixed rules in goroawase.
There are instances of goroawase being used in anime and manga. In Soul Eater, for example, the number to call Shinigami is 42-42-564, which reads shini-shini-goroshi, meaning “death-death-murder”. Goroawase appears to be used in One Piece as well, though I’ve never read the manga.
Hope that helps!