iPhone sends texts to landline instead of mobile number

I work for an emergency restoration company and I’m trying to send text messages to my clients keeping them updated on the storm. My iPhone keeps sending messages to the clients landline. I have tried a new text thread and it continues to still send to the landline. I have deleted the landline number and keep the mobile number and it went through but the moment I add their landline back it sends messages right to their landline. Being in emergency restoration I need my phone to do what I’m asking it to do. Can someone help me figure this out.