TI Silent 700 was the standard for decades.
And, it was one of the uses that popularized the Radio Shack Model 100
(also the other kyocera variants, and the Epson HC20/HX20)
TDDs (Telecommunication Device fpor the Deaf) included many portable ones,
mostly using "Baudot"/"Murray"/CCITT-2
The Atari Portfolio, Poqet, and HP95/100 models briefly became the
standard pocket terminals.
But there have been many more over the years, such as the Epson RC20
wristwatch (with sorta-Z80, RAM, ROM, serial port),
Palm (and Fossil Palm wristwatch), Ipaq, etc.
How hard is it to add a serial port to a "smartphone"?