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Can VoWifi use the internet connection over the second SIM?

Backstory: a while ago I was traveling abroad. The roaming costs are astronomical. As my ISP provides a VoIP (SIP protocol) service, I installed a SIP client app and configured it to use my ISP account. As it uses internet to communicate, I could use WLAN or the UMTS/LTE network with a local SIM in my phone (I removed my original SIM). So I could make and receive calls on my original phone number without any hassle, for low cost.

Back to today: SIP is phased out and "replaced" with VoWifi.

The question: if I am abroad, and there is no WLAN available, can I use a second (local, read: cheap) SIM (in a dual SIM phone) to connect to the internet over UMTS or LTE and use it to make and receive calls with my original phone number (using VoWifi)? Or is VoWifi strictly WLAN-only?

In other words: can I have effortless and cheap connectivity for my regular phone number abroad, like I could using SIP?

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  • VoWifi always uses WiFi only - therefore the name. If voice calls are transmitted over 4G it is called VoLTE instead. Or are you referring to just VoIP? The latter is usually available via Internet and is not restricted to certain special networks or a VPN tunnel.
    – Robert
    Feb 23, 2021 at 15:09
  • @Robert I am referring to the function that is advertised as "VoWifi" or "WifiCalling". It is available on many phones and supported by many providers. What I am asking is, can this very feature use mobile internet over a second SIM (which is much cheaper when abroad). As far as I know, VoLTE always uses the mobile internet connection over the same SIM that "holds my phone number", which means: very expensive when used abroad.
    – David Balažic
    Feb 23, 2021 at 16:02
  • As "VoWifi" or "WifiCalling" is not standardized it depends on the company providing this service. If WifiCalling is available on any Internet connection it should also work using a mobile connection (some mobile network providers limit WifiCalling to special Wifis/networks).
    – Robert
    Feb 23, 2021 at 16:13
  • @Robert The underlying standards Generic Access Network (GAN) and Unlicensed Mobile Access (UMA) are over 10 years old. Almost all (modern) smartphones support it. It is definitely standardized.
    – David Balažic
    Feb 23, 2021 at 17:06
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    Sorry, my last comment was not accurate enough. I was not talking about the used protocol, I was talking about the limitations that may be applied by your provider. Those are not standardized, hence a mobile network provider advertising "WifiCalling" may mean that it is accessible from every network worldwide or it could mean that it is only available on Wifi hotspots of the same network provider or a VPN is required or completely different limitations apply.
    – Robert
    Feb 23, 2021 at 17:12

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