Gmail web interface shows 4 contacts online, but Ktp only shows the mobile icon in one of them.
What do you mean? Does GMail web interface show that these 4 online contacts are connected with a mobile device? I'm not sure about what happens if you have at the same time a mobile and a non-mobile client connected, perhaps in this case gmail shows the phone icon, while we don't... Anyway the "mobile" presence comes from telepathy, we don't detect it, we just show it, therefore, if you think that there is a bug, it should be reported upstream ( https://bugs.freedesktop.org/ )
As per a discussion on irc between jcarlos and d_ed, Google itself is not sending the mobile presence indication for the new Hangout accounts.