Rei suddenly have smartphone & LINE?
It's quite commonplace for mangaka, or any comic authors anywhere in the world, to keep their long-running series more or less contemporary. This manga launched in 2007, so everybody had feature phones and were even using public telephones. However, about a chapter a month makes the time in RL advance much faster than the time in manga, unless it's some kind of crazy time skip manga. Keeping the manga world more or less similar to the surrounding reality can help with casual reader experience. Naturally not every manga could ever allow it, if it's about something even remotely tied to a time period, even if the time period was nothing but the early 2000's. World has changed a lot from that already.
This manga, however, isn't strictly tied to time in that sense, so the mangaka had the option to keep this contemporary. If you think of slice of life, in those it's even easier, as quite often time doesn't progress in them at all. For example, in Minaki-ke seasons come and go, repeatedly, but the main characters are frozen in time, always remaining the same age and same year in school.