Some times its better to KISS.
Why not ask the customer about the two way pagers. I have one that receives a message and I can respond.
The AA batteries in it last for a month and the person would have a record saved of what was requested.
Mine is $10 a month for an alpha/numeric and it would take a long time to meet the cost of a lot of horns and labor to hear a hoot and hollar.
They could probably get a 30 day trial to see if it would work for them. Any computer with internet access can send a text message, any phone could send a coded message. (111 call the office, 222 come to the office, 333 meet customer at shipping platform, etc.)
