I think you mean congestion, presumably for "over the top" calls (WhatsApp for example) that use ordinary mobile data. StarHub's 4G network in Singapore has very good coverage (reception). Traditional voice call completion rates should be quite high since VoLTE is prioritized.
In my line of work, I conduct a lot of home visits. I mean reception. Discovered some residential areas had 1 bar coverage on a relatively new iPhone 15, so was surprised to encounter that. And it was not one off - it was observed over a few weeks.
And I mean voice calls direct to the mobile number. Not OTT calls.
I tolerated the worsening deprioritisation for as long as I could as I still had credits remaining. But it became evident I had to port out when I missed some crisis calls. As a control, it's the same phone, no concerns on CL 5G.
Thank you for trying to reframe.