Messages automatically sent to landline instead of mobile numbers

My company recently activated a new business texting solution for our customers which allows them to text our company landline. Messages are now automatically being sent to the landline rather than the mobile number. I am under the impression that Apple uses the last number used for the contact, so why are messages not sent to the mobile number that our customers have been primarily using? For example, I have a contact with two numbers. The first number listed for the contact is Mobile. The second number listed for the contact is Work which is a land line. Since we activated this capability, all messages are sent to the land line.