How do I choose my phone's default SIM?

With two lines on one phone, you choose which one does what.

iPhone: in Settings → Mobile Service, set your Default Voice Line and Mobile Data line. Tap each line to label it and switch it on or off.

Android: in Settings → Network & internet → SIMs (the wording varies), choose the preferred SIM for calls, texts and mobile data.

Switch off Allow mobile data switching if you want data to stay on olilo.