On desktop/laptop computers, "Speaker" button should either allow choosing the output device (if multiple are present) or be labelled "Deafen"
I've tested Calls 42.rc.1 with a SIP "echo test" call on my Fedora 35 desktop computer, and the "Speaker" does nothing, at least not with my USB sound card.
Arguably, my computer has a complicated collection of sound devices, where the UMC202HD USB sound card serves for both the microphone input and the output (be it for speakers or headset; yes, it has output for both simultaneously, with physical volume knobs to control them independently) and I turn off all the other devices in pavucontrol
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The "Speaker" button probably doesn't linguistically make sense on devices other than phones (that have an earpiece). Unless you can make it into an output device chooser when there is more than one active output device available, it should probably be labelled "Deafen" otherwise; but even then I'm not sure why one would do that vs putting the call on "Hold"...