The sms_client port (/usr/ports/comm/sms_client) has the ability to send SMS messages to subscribers of AT&T, however the URL to send SMS to AT&T subscribers has changed, in the file /usr/local/etc/sms/services/att_web
It has the following:
SMS_url = "http://www.mobile.att.net/mc/mc_pagersend.cgi"
that URL is no longer available and you cannot send SMS through it.
Change SMS_url to:
SMS_url = "http://www.mymmode.com/messagecenter/pagersend.cgi"
I have tested this with my phone and have been able to get all of the SMS messages sent via sms_client to my AT&T wireless phone
Patch implementing your suggestion was committed.