Mobile communication can use 3 channels:
* WW: wifi
* DD: mobile data
* VV: voice network including GSM, CDMA and LTE. SMS uses the same network, and can’t use wifi or mobile data.

Flight mode on phones (or laptops) disables all of them but you can re-enable WW, as tested on my Vivo1820

— to Reduce roaming cost … avoid DD + calls.

Flight_mode is overkill. Incoming_SMS_on_roaming is often free.

WW doesn’t depend on SIM card or roaming.

— SIM card (including roaming) … provides DD/VV not WW. In other words, DD/VV require SIM cards. Computers and tablets without SIM cards can use WW not DD/VV. So WW is one common technology available on all phones and computers.

— Whatsapp/Wechat calls … use WW/DD not VV. In contrast, many smartphone apps can launch voice calls.. either VV call or VOIP call. The VV call would require a SIM and take up your precious airtime.

* Internet-enabled mobile APPs require either WW or DD and may occassionally use SMS (requires VV network)
* Internet-enabled laptop applications require WW (or LAN) and never require DD/VV.