Modify AT&T's suggestion for using pppd to connect to their wireless network.
http://www.att.com/esupport/article.jsp?sid=36059&cv=820#fbid=Ph3zROJiN9D
Tested on beaglebone with sierra 250 from Ting. Basically, one just
dials ATD*99***1#
and everything works. On older networks, use the
777 number instead.