contacting a carrier on there personal phone

Are there any union rules on "having" to use your cell phone for work related purposes? Let me clarify. I would rather my management contact me using the scanner instead of using my phone. My position, is that they do not pay my cell phone plan (yes, I am paid by the USPS, but not like a union credit card or a company benefit), so running up my data sending multiple images of package scans is a problem. I've said to them that I may not have a phone. I had an issue weeks ago when it ran out of battery and came back to office after having done OT. I also don't always carry a phone charger, my route truck has been in service for about a month now.