One economical strategy for international roaming that works for many people is simply to get a separate roaming data eSIM if your phone supports eSIMs. For example, according to esimdb.com Keepgo is selling a 1.1GB (total using an esimdb.com promo code) eSIM with no expiration (!) for US$3 that works in 153 countries/territories including weird ones like Afghanistan. Or US$2 if you want the 100 country/territory version. A 1.1 GB allotment lasts a very long time if you’re simply using WhatsApp text messages and summoning a ride using Grab or an equivalent app — then quickly shut off mobile data once your ride arrives. (And you otherwise stick to Wi-Fi for app downloads, downloading offline maps, email, video, audio, etc. And set your phone to low data mode.) Meanwhile, while you’re in Singapore, pay only $3.30 per month for Circles.Life (current deal), for example.
In other words, even if you travel internationally, the best overall deal may not be with a local plan that offers much or any international roaming. The lowest cost local plan could still work best for you.
I should also point out that some carriers support VoWifi (”Wi-Fi Calling”), and they may not charge any extra for that service when you’re connected to a Wi-Fi network outside Singapore (but check that). When you’re connected to a hotel, coffee shop, or airport Wi-Fi network your phone can support voice calls and SMS text messages via VoWifi. Incoming SMS text messages via VoWifi should always be free, even for carriers that charge international roaming fees for VoWifi use overseas.