Messages: If Wi-Fi internet is available, are SMS/MMS messages sent over the internet?

"Messages" is an app that uses a number of different kinds of protocols. It uses the cellular network for those types of messages that require it.

SMS requires a cellular voice connection. However, it doesn't require a data connection. That's why flip phones can send SMS messages.

MMS messages require a cellular data connection. Group messaging also requires a cellular data connection. That's why your group messages with Android users won't work other without a cellular data connection.

SMS messages don't require a data connection at all. MMS messages require a cellular data connection. iMessages (and Messenger, Whatsapp, etc) can use either cellular data or WiFi

markbsigler wrote:

In our group there are Android devices, not capable of iMessages, so Messages does delivery for all by MMS which requires cellular data network. This is what I was looking to confirm.

Yes, that's correct.

For mixed groups, I also prefer a dedicated group messaging app.

Which messages? iMessage does bypass cell voice and uses only cell data or WiFi depending on what you are connected to at the time.

markbsigler wrote:

If Messages doesn't require cellular network to deliver SMS or MMS messages, then there seems to be a chronic issue with groups that include a non-Apple device (Android).

I think there's some confusion over terminology. "Messages" in an app. "iMessage" is a messaging service. The Messages app requires a cellular network to send SMS and MMS messages. It does not require a cellular network to send iMessages.

If everyone in the group is using iMessage, you will not require a cellular data network. If anyone in the group is not using iMessage, you need a cellular network.