You were only one section away, look at (c) below. I had forgotten it only applies in the 800MHz band. While googling for this info, I also found petitions to the FCC to apply this rule in the 700MHz band.
Sec. 90.647 Station identification.
(a) Conventional systems of communication shall be identified in
accordance with existing regulations governing such matters.
(b) Trunked systems of communication, except as noted in paragraph
(c) of this section, shall be identified through the use of an automatic
device which transmits the call sign of the base station facility at 30
minute intervals. Such station identification shall be made on the
lowest frequency in the base station trunk group assigned the licensee.
Should this frequency be in use at the time station identification is
required, such identification may be made at the termination of the
communication in progress on this frequency. Identification may be made
by voice or International Morse Code. When the call sign is transmitted
in International Morse Code, it must be at a rate of between 15 to 20
words per minute and by means of tone modulation of the transmitter, the
tone frequency being between 800 and 1000 hertz.
(c) Stations operating in either the 806-824/851-869 MHz or 896-901/
935-940 MHz bands that are licensed on an exclusive basis, and normally
employ digital signals for the transmission of data, text, control
codes, or digitized voice may also be identified by digital transmission
of the call sign. A licensee that identifies its station in this manner
must provide the Commission, upon its request, information sufficient to
decode the digital transmission and ascertain the call sign transmitted.