If u are planning to use Digi + iPhone, here's what you need to do to enable "VoWiFi" using another SIM card :
1. Once reach SG , enable manual network selection on the Digi card and manually select TPG / M1 which Digi can't roam into
2. Ensure you have another SIM with Mobile Data enabled and did not block IPSec VPN (China Mobile do this)
3. Wait for a few seconds , you should see "Digi using Mobile Data" , means Digi is now connected to VoWiFi using your mobile data. Call and SMS rate are the same as local in this situation

I used this before in Taiwan when calling back to Maybank for checking the payment details , it works very well.