An iPhone 8 Plus can't send / receive MMS messages (i.e. non-iMessage group messages) when on WiFi calling on Verizon – it just sits with the progress bar when trying to send and then eventually fails.
Phone calls and individual SMS messages work fine when on WiFi calling, and going down the street to where there's 4G signal causes MMS to work immediately.
A Galaxy S9+ on the same WiFi calling is able to send/receive fine. The underlying internet is 1Gbps fiber and works well.
In case it's relevant, there is extremely marginal cell tower coverage at this location (i.e. ~1 bar).
Updating iOS on the phone did not solve the issue.
Best Answer
This is expected.
WiFi calling has no provision for sending either SMS or MMS messages that’s a function of the GSM Network. Also, MMS is multimedia messaging, not “non-iMessage group messages.” Now, as long as you’re connected to WiFi, you can receive iMessage messages.
It is relevant if you have your cellular on, but signal is so low that you’re using WiFi calling instead. SMS using a very small amount of bandwidth compared to MMS, so it’s entirely possible regular SMS messages go through and MMS messages do not. Also, unlike SMS, MMS will typically not queue up and continue resending; they usually delete after a period of time, sent or not.