At least with the netdevice, because the custom AT command for retrieving the IP configuration doesn't support IPv6 at all, and the devices don't support DHCP on the net interface.  I believe IPv6 can be used with PPP on these devices, but since the hso plugin doesn't use PPP at all, IPv6 attempts fail with:

error: couldn't connect the modem: 'GDBus.Error:org.freedesktop.ModemManager1.Error.Core.Failed: Call setup failed'

[mm-at-serial-port.c:408] debug_log(): (ttyHS0): --> 'AT+CGDCONT=5,"IPV6","epc.tmobile.com"<CR>'
[mm-at-serial-port.c:408] debug_log(): (ttyHS0): <-- '<CR><LF>OK<CR><LF>'
[option/mm-broadband-bearer-hso.c:618] authenticate(): Not using authentication
[mm-serial-port.c:958] mm_serial_port_open(): (ttyHS0) device open count is 3 (open)
[mm-serial-port.c:1003] mm_serial_port_close(): (ttyHS0) device open count is 2 (close)
[mm-at-serial-port.c:408] debug_log(): (ttyHS0): --> 'AT$QCPDPP=5,0<CR>'
[mm-at-serial-port.c:408] debug_log(): (ttyHS0): <-- '<CR><LF>OK<CR><LF>'
[mm-serial-port.c:958] mm_serial_port_open(): (ttyHS0) device open count is 3 (open)
[mm-serial-port.c:1003] mm_serial_port_close(): (ttyHS0) device open count is 2 (close)
[mm-at-serial-port.c:408] debug_log(): (ttyHS0): --> 'AT_OWANCALL=5,1,1<CR>'
[mm-at-serial-port.c:408] debug_log(): (ttyHS0): <-- '<CR><LF>OK<CR><LF>'
[mm-serial-port.c:1003] mm_serial_port_close(): (ttyHS0) device open count is 1 (close)
[mm-at-serial-port.c:408] debug_log(): (ttyHS0): <-- '_OWANCALL: 5, 3<CR><LF>'
[mm-bearer.c:461] connect_ready(): Couldn't connect bearer '/org/freedesktop/ModemManager1/Bearer/0': 'Call setup failed'

This brings up something I was thinking about during the IP-family discussion; if a device can do IPv6 via PPP but not ethernet, if the bearer requests IPv6, should we create a PPP bearer?  Or should we simply mask out IPv6 on devices that can't support it on the main bearer, like hso?