From the AT command AT+CLAC, which lists all the available commands supported by the module, I can’t find the +CUSD command in the list. It might be that the module does not support USSD (See attached image; quite strange due to the fact that when entering AT+CUSD? and AT+CUSD=? we get valid outputs). What we get from AT+CLAC is:
&C
&D
&E
&F
&S
&V
&W
E
I
L
M
Q
V
X
Z
T
P
\Q
\S
\V
%V
D
A
H
O
S0
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S30
S103
S104
+FCLASS
+ICF
+IFC
+IPR
+GMI
+GMM
+GMR
+GCAP
+GSN
+DR
+DS
+CMEE
+WS46
+PACSP
+CFUN
+CLCC
+CBST
+CRLP
+CV120
+CHSN
+CSSN
+CREG
+CGREG
+CEREG
+CSCS
+CSTA
+CR
+CEER
+CRC
+CGDCONT
+CGDSCONT
+CGTFT
+CGEQREQ
+CGEQMIN
+CGEQOS
+CGCONTRDP
+CGSCONTRDP
+CGTFTRDP
+CGEQOSRDP
+CGQREQ
+CGQMIN
+CGEREP
+CGPADDR
+CGDATA
+CGCLASS
+CGPIAF
+CGSMS
+CSMS
+CMGF
+CSAS
+CRES
+CSCA
+CSMP
+CSDH
+CSCB
+ES
+ESA
+CMOD
+CEMODE
+CVHU
+CECALL
+QCGEQMIN
+QCGEQOS
+QCGEQREQ
+QCGQMIN
+QCGQREQ
+QCGTFT
+CFUN
$QCPWRDN
+CMUX
+IPR
+QNAND
+QWSERVER
+QWTOCLIEN
+QWTOCLI
+QWPARAM
+QDATAFWD
+QFTCMD
+QBTPWR
+QBTLEADDR
+QBTNAME
+QBTGATREG
+QBTGATSS
+QBTGATSC
+QBTGATSD
+QBTGATSIND
+QBTGATSNOD
+QBTGATRRSP
+QBTGATWRSP
+QBTGATSENLE
+QBTGATADV
+QBTGATDISC
+QBTGATPER
+QBTGATDBALC
+QBTGATDBDEALC
+QBTGATSA
+QBTGATDA
+QFCT
+QIIC
+QAUDLOOP
+QDAI
+CMUT
+CLVL
+CRSL
+QSIDET
+QAUDMOD
+QEEC
+QMIC
+QRXGAIN
+QAUDRD
+QAUDPLAY
+QAUDSTOP
+QPSND
+QTTS
+QTTSETUP
+QLTONE
+QLDTMF
+QAUDCFG
+QTONEDET
+QWTTS
+QPCMV
+QTXIIR
+QRXIIR
+QPOWD
$MYPOWEROFF
+QSCLK
+QCFG
+QADBKEY
+CBC
+QADC
+QADCTEMP
+QGPSCFG
+QODM
+QFUMO
+QFUMOCFG
+QPRINT
+QSDMOUNT
+QFASTBOOT
+QPSM
+QPSMCFG
+QLINUXCPU
+QVERSION
+QSUBSYSVER
+QTEMPDBG
+QTEMP
+QTEMPDBGLVL
+QDIAGPORT
+QLPMCFG
+QSGMIICFG
+QWWAN
+QLWWANUP
+QLWWANDOWN
+QLWWANSTATUS
+QLWWANURCCFG
+QLWWANCID
+QLPING
+QWIFI
+QWSSID
+QWSSIDHEX
+QWAUTH
+QWMOCH
+QWISO
+QWBCAST
+QWCLICNT
+QWCLIP
+QWCLILST
+QWSTAINFO
+QWCLIRM
+QWSETMAC
+QWRSTD
+QWIFICFG
+QAPRDYIND
+QFOTADL
$QCSIMSTAT
$QCPBMPREF
$CREG
$CCLK
$QCCNMI
*CNTI
$QCCLR
$QCDMR
$QCDNSP
$QCDNSS
$QCTER
$QCSIMAPP
$QCPINSTAT
$QCPDPP
$QCPDPLT
$QCPWRDN
$QCDGEN
$QCPDPCFGE
$QCANTE
$QCRPW
$QCSQ
$CSQ
$QCSYSMODE
$QCCTM
$QCBANDPREF
^PREFMODE
^SYSINFO
^SYSCONFIG
^CARDMODE
^DSCI
$QCVOLT
$QCHWREV
$QCBOOTVER
$ECALL
$QCDEFPROF
$QCMRUE
$QCMRUC
$QCAPNE
$QCPDPIMSCFGE
$QCCLAC
^SPN
$QCRMCALL
$QCDRX
$QCRSRP
$QCRSRQ
$QCACQDBC
$QCATMOD
$QCCOPS
^MODE
$QCSIMT
$QCNSP
$QCRCIND
$QCCSGCOPS
$QCPDPCFGEXT
$QCPRFCRT
$QCPRFMOD
OK
