The UA timetable for August shows UA metal flights using ranges 1-1299 and 1400-1764.
Though in reality they can use 1-1339 and 1345-2049. 1340-1344 are listed as ExpressJet Extra Sections and the UA Extra Sections begin at 2050.
http://www.united.com/CMS/UADocument.../timetable.pdf Page 13