Dial up worked fine in 1607. With the update to Creators' build it now reports "error 633 - com port in use."
The modem has always been on COM 3. Tried ports 5, 9, 11. Same. COM 1, the only port listed in Device Manager, is shown as "in use." Process Explorer shows no info when queried for "serial."
Searched, read and tested virtually every suggested fix...
Uninstalled and reinstalled dial-up connection
Uninstalled modem (in Safe Mode), booted normally and updated modem driver
Created a "reservedport" registry entry for port 1723
Ran netstat -aon search for port 1723; it's not listed
Disabled the broadband adapter temporarily and turned off the broadband modem temporarily
SFC /scannow shows no errors.
There are no VPN connections on this PC.
This isn't a modem fault; MS FAX works fine.
There was no DUN failure when 1511 O/S was updated to 1607.
Applied Creators' update to several 1607 restored images and over a 1511 restore as well.
DUN had been functional on them all until the 1703 update.