Doesn't all SG telco requires VoLTE now ?My guess is possibly due to poor connection.
Simba calls need VoLTE so that could be the reason.
Can try restarting your phone.
For me, I got another telco line for my dual SIM phone to provide data support. Perhaps not 100% foolproof but that's my current see-how solution, since I don't make nor receive many calls.
