MC7455 not dialing calls

I’ve got an MC7455 that connects to LTE and I can use the data.

But, when I go to dial a call it says NO CARRIER and the +ceer is EMM detached.

I’m on Sprint (via Tello) using the Generic firmware. I can’t connect via the Sprint firmware.

The same issue presents itself if i use the AT&T firmware and an AT&T SIM.

I get into the terminal with minicom -D /dev/ttyUSB2 and run this:

+CEER: EMM detached

Some informational output: ``` ati Manufacturer: Sierra Wireless, Incorporated Model: MC7455 Revision: SWI9X30C_02.30.01.01 r7792 CARMD-EV-FRMWR2 2018/07/13 02:24:52 MEID: 35907206321180 IMEI: 359072063211805 IMEI SV: 16 FSN: LQ842740430210 +GCAP: +CGSM

Current Time: 2385 Temperature: 24
Reset Counter: 3 Mode: ONLINE
System mode: LTE PS state: Attached
LTE band: B41 LTE bw: 20 MHz
LTE Rx chan: 41094 LTE Tx chan: 41094
EMM state: Registered Normal Service
RRC state: RRC Idle
IMS reg state: No Srv

PCC RxM RSSI: -78 RSRP (dBm): -110
PCC RxD RSSI: -94 RSRP (dBm): -138
Tx Power: – TAC: 1F0F (7951)
RSRQ (dB): -11.8 Cell ID: 03FAA401 (66757633)
SINR (dB): 8.2


+CNUM: “Line 1”,“817*******”,161


Hi @nunziotocci, the 7455 modems are data modems. “atdxxxxxxxxxx;” which is for starting a voice call with return an error. You can get the modem’s technical specifications from

Then what is the purpose of the PCM/I2S audio interface? See page 61 of the Technical specifications.

And if the MC7455 is truly not a voice modem, what modem does Sierra offer that has both LTE and voice?

That is surprising and disappointing. I chose this module after I referenced the document “AirPrime EM74xx-MC74xx AT Command Reference” and the AT command for dialing said it was supported.

We are really stuck now. We need a modern voice modem for the Sprint network that will work for a while. An LTE modem seemed like it was the only one that wouldn’t become obsolete in the next year or so. If anyone has a recommendation we’d appreciate it.

@nunziotocci, the PCM/I2S signals are part of M.2 and so they are listed even though the 7455 modem does not support them
@justintocci, Modems are certified with SPRINT as data only (no voice). For other carriers, you can get voice. Please reach out to our distributors or sales team and they can provide more information.

Is there another modem that support voice calls dear @anon56110884

how about using WP76xx modules?

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