I know it wont' work with letstalk, target, and radioshack because they do not let you transfer the equipment to other line. They consider that as "deactivation" and charge you an ETF.
However, Amazon's wording is a little different. It says
"If you transfer this equipment to another carrier's service or to another line in your family account and deactivate the line that Amazon established for your device"
I'm assuming as long as I maintain the dumbphone line, I can transfer the equipment to other line. Does anyone have experience with Amazin Wireless?