We have cell phone "smartphone" with data plan (no voice plan). If, I use Krown pocketcom to hook up with our smartphone. To call someone's tty or relay service. It will cost 20 or 40 cents per minutes. No thank to pay big billing.
http://www.payphone.com/shop/instruc...pocketcomm.pdf
Krown PocketComm Portable TTY, VCO & HCO
Krown Manufacturing, Inc.| FAQ
I seem that Krown pocketcomm is not popular. Not see any item at Krown's store online. Probably, they are no longer productiion
*****DISCONTINUED ITEM******