imessage keeps deleting my phone number

I have a work phone and a personal phone and use the same apple account on both. This allows me to use imessage from my work phone and my friends don't know the difference b/c it comes from the same apple id. I tend to leave my personal phone off and use my work phone 90% of the time. Lately, imessage has started removing my personal phone number as an option under imessage on my work phone. When I turn my personal phone back on everything gets added back, but I don't want to have to turn it on at all. Why does imessage keep removing my personal phone number as an option?