We shouldn't rely on the whole connection sequence to finish before setting the port as connected, because sending AT commands between ATD and the PPP session establishment may end up interfering. So, try to flag the port as connected just after ATD is sent; and if ATD fails reset the flag right away so that we can run +CEER. See https://lists.freedesktop.org/archives/modemmanager-devel/2017-January/003773.html
Discussed here: https://lists.freedesktop.org/archives/modemmanager-devel/2017-March/004257.html
Fixed in git master, mm-1-6 and mm-1-4.
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