Seems like there are a few other things that are not SMS, including SMS itself:
$ dig is-not-s.ms @sym.noone.org. axfr
; <<>> DiG 9.11.5-P4-5.1+deb10u7-Debian <<>> is-not-s.ms @sym.noone.org. axfr
;; global options: +cmd
is-not-s.ms. 86400 IN SOA sym.noone.org. hostmaster.noone.org. 2018092900 28800 7200 604800 86400
is-not-s.ms. 86400 IN NS ns4.beckert.ch.
is-not-s.ms. 86400 IN NS sym.noone.org.
bloggen.is-not-s.ms. 86400 IN CNAME sym.noone.org.
blogging.is-not-s.ms. 86400 IN CNAME sym.noone.org.
changelog.is-not-s.ms. 86400 IN CNAME sym.noone.org.
e-mail.is-not-s.ms. 86400 IN CNAME sym.noone.org.
email.is-not-s.ms. 86400 IN CNAME sym.noone.org.
etherpad.is-not-s.ms. 86400 IN CNAME sym.noone.org.
icq.is-not-s.ms. 86400 IN CNAME sym.noone.org.
im.is-not-s.ms. 86400 IN CNAME sym.noone.org.
irc.is-not-s.ms. 86400 IN CNAME sym.noone.org.
jabber.is-not-s.ms. 86400 IN CNAME sym.noone.org.
sm.is-not-s.ms. 86400 IN CNAME sym.noone.org.
sms.is-not-s.ms. 86400 IN CNAME sym.noone.org.
usenet.is-not-s.ms. 86400 IN CNAME sym.noone.org.
is-not-s.ms. 86400 IN SOA sym.noone.org. hostmaster.noone.org. 2018092900 28800 7200 604800 86400
;; Query time: 672 msec
;; SERVER: 178.63.92.236#53(178.63.92.236)
;; WHEN: Sun Aug 28 15:27:12 BST 2022
;; XFR size: 17 records (messages 1, bytes 441)