Everyone keeps a cell phone handy, and I think if you explained the issue and asked your employees if they'd be ok with having the authentication go to their personal cells most probably would be ok with it. They probably have it flipped out all day anyway! If you're concerned, for team members who regularly use their personal cell phones for work we offer a monthly stipend ranging depending on the need and their role. Managers who do so much on their phones, and Maintenance team members who use their phones to receive emergency calls, close out work orders, etc., would receive a higher monthly amount than someone only using a cell phone for the 2-factor process. If upgrading your phone system and adding direct dial extensions would be your alternative, compensating for individual phone usage sounds better.