Why did my 住民税 suddenly jump this June? (first-timers, read this before you panic)

Got my residence tax (住民税) bill this week and it's noticeably higher than last year. Took me a while to realize: it's based on LAST year's income, billed from June. So if you earned more in 2025, this June's bill reflects it. Classic trap: first-year arrivals often get surprised in their SECOND June, because year one you had little/no prior-year income. Question for the group: can you pay in installments (分割)? My ward office phone line is always busy. Anyone done it?