Crossed Wireless Connections

[JM02871]

I've just received a call from an AT&T cell phone subscriber who stated that my cell phone number with Verizon is, in fact, his voice mail call-in number. I've had my Verizon number for over 10 years and have heard this complaint a few times before. 

What is the solution and what should I tell the AT&T subscriber about the issue? 

Thank you.