The landline automatically replies to text messages

First ask Huawei, Cisco, ZTE, etc. for technical information on IP phone switches, and ask them for IP phone network solutions. They will tell you to go to China Mobile, China Telecom, and China Unicom to purchase SMS sending ports. Then just integrate it into the control end of the switch.

If you think the products of large companies are expensive, there are many small companies you can consider, such as Shenou, Huguang, Guowei, etc. I believe their services will satisfy you.

Attached is the relevant product information, you can also learn about it yourself first.