Account uncertainty with a prepaid service

Earlier in the month a family member switched devices while staying on the same prepaid service, and that step created uncertainty about which phone actually holds the active line. Some texts arrive on one device and some on the other, leaving everyone unsure which number friends should keep. The refill history looks complete, yet the online account page lists only limited details about the current handset. Having two partially active phones in the mix makes planning simple things like meeting times or pickups much harder than necessary. Has anyone faced this kind of overlap, where the line appears to float between devices, and figured out a clear way to demonstrate the issue when asking for help?