I recently switched to my wife's old iPhone 5 from my iPod touch g5, so that I could use iOS 10. Neither my iPod touch g5 nor my iPad mini g1 are compatible with iOS 10 so those devices go in the retired pile.
To use the iPhone 5 while mobile, I installed my Google fi data-only SIM. When Apple's setup routine was configuring the phone, it asked me to add the data SIM's phone number to my Apple ID / iMessages / FaceTime and I approved.
I didn't know at the time that it would cause some of my conversations to become divided and create more confusion for me and my messaging friends.
To resolve this conflict & confusion, I opened the settings on my iPhone 5 and then Phone options in settings. There, I set my phone number to match my actual number, which f course, is used by my regular mobile (Nexus 5X). Next, I went into the options for Messages, then Send & Receive, and unchecked the option to use the unwanted new data-only phone number. Next, went into FaceTime options and did the same with Send & Receive. This makes it so that all messaging to & from the data-only device, the iPhone 5, is associated with my email address instead of my mobile/phone number.
Tested and determined that all my conversations were again, routed to their correct conversation threads.