update: i've found a lot more information online by googling "tamagotchi connecting to cellphone" in japanese!
firstly, this article, on the tamatalk domain itself:
this basically confirms the information on the rest of the articles i've found today except for a slight inconsistency, which i'll get to later. just as i suspected, the app is downloadable for normal cellphones and is likely included in the special tamagotchi one. the article also talked about a tamagotchi game for cellphones which may or may not be related to this, where you can apparently play fortune-telling games with your tamagotchi.
next, another article in english:
the aerial part is likely not true since that's never been used as anything more than a piece of plastic to symbolize the connection capabilities, however i think by now you've noticed the inconsistency: some online sources talk about the tamagotchi connecting to the phone using infrared, and others (such as the tamagotchi wiki) mention a log-in/log-out code system. maybe it used different systems for different game modes? the game mode where you can collect food from each region of Japan is very interesting, especially considering the fact that it seems to use a password system similar to the western v3, or the "souvenir collecting" feature of the v5 tamatown.
according to this guide, the keitai has different options for connecting to a cell phone:
regardless, now let's look at a
japanese article published in october 2004, that i found and translated using google translate. this has got to be the most extensive information online regarding this feature, and it answers some of our questions nicely.
this sentence seems to imply that the tamagotchi connected to cellphones using infrared, which basically brings up as many questions as it just answered, since it contributes to the inconsistency between articles. however, i'd trust this source more since it's the local media of the time and not an online article written a decade later.
as we can see, just like the first article, this confirmed my suspicions: the app is indeed downloadable. this part also shows us some of the features, which seem very intriguing, almost like a portable version of tamatown! let's just hope that the app has been archived, since it's probably just a simple executable that's likely been written in J2ME like most mobile games of that era.
now THIS is one of the questions i wanted answered. not only does it show us the date when the site was opened (which means we may be able to still access the game's download page using the wayback machine), but it also contains a comprehensive list of the supported phone models. this means that we can research these phones further, in order to find out if they all include infrared and on which architecture they run so we can find out more about the app. also, now that we know the exact supported models, my plan of purchasing random phones on japanese auction sites and hoping that they have the game installed (or at least having the reassurance that, if the game is found some day, then it'll be able to run on it) may work! maybe this whole endeavor will be a new mission for the tamagotchi community, up there with recovering all of tamatown?
i'm very excited to see what comes of this in the future!