Here's some code. Yep, pretty much you got it. In the last two lines, trying to search for where the from room and to room of a door match a room in a particular phase. Here my (lazy) variable name was called phaser, which is passed in as a Phase element from Revit.
Happy Coding!
Element elem = doc.GetElement(modifiedElemId);
string phase = elem.GetParameters("Phase").FirstOrDefault().AsValueString();
//Phase phase2 = elem.GetParameters("Phase").FirstOrDefault().Element as Phase;
ElementId phaseid = elem.GetParameters("Phase Id").FirstOrDefault().AsElementId();
Phase phaser = doc.GetElement(phaseid) as Phase;
//TODO consider implementing Parameter Filters instead of pulling items here to LINQ
//IEnumerable<Autodesk.Revit.DB.FamilyInstance> enmu2 = ls.Where(x => x.get_ToRoom(phaser) != null && x.get_FromRoom(phaser) != null);
//IEnumerable<Autodesk.Revit.DB.FamilyInstance> enmu4 = ls.Where(z => z.Name.StartsWith("DR"));
IEnumerable<Autodesk.Revit.DB.FamilyInstance> enmu3 = ls.Where(w => w.GetPhaseStatus(phaseid) == ElementOnPhaseStatus.Existing || w.GetPhaseStatus(phaseid) == ElementOnPhaseStatus.New || w.GetPhaseStatus(phaseid) == ElementOnPhaseStatus.Temporary);
IEnumerable<Autodesk.Revit.DB.FamilyInstance> enmu2 = enmu3.Where(x => x.get_ToRoom(phaser) != null || x.get_FromRoom(phaser) != null);
IEnumerable<Autodesk.Revit.DB.FamilyInstance> enmu = enmu2.Where(y => y.get_ToRoom(phaser).Id == modifiedElemId || y.get_FromRoom(phaser).Id == modifiedElemId);
