[Python] Why does my modem send a DTMF 9 tone three times even when AT+VTS is only called once?

I'm working on a project where I use a StarTech USB56KEMH2 modem with Python (PySerial) on Linux to call my building's buzzer (MICROM system) and open the door by sending a DTMF 9 tone.

The modem is set to voice mode (AT+FCLASS=8) and auto-answers after 1 ring (ATS0=1). Once the call is active, I send the following DTMF tone:

AT+VTS={9,8}


(I also tried AT+VTS=9 and got the same result.)

According to the documentation, this should play a 9 tone with a duration of 80 ms. And indeed, it works — the buzzer opens the door, but there's a weird issue: the tone is heard or interpreted three times, as if I had pressed 9 three times.

What I’ve tried

• Verified that AT+VTS is called only once per call (using logs).
• Reset the modem (ATZ, ATE0, etc.) between each call.
• Changed tone durations using both classic and {digit,duration} format.

Command Result
AT+VTS=9 Works but repeats 3 times
AT+VTS={9,10} Works but repeats 3 times
AT+VTS={9,8} Works but repeats 3 times
AT+VTS={9,7} Works but repeats 3 times
AT+VTS={9,6} No sound / not detected
System setup

• StarTech USB56KEMH2 modem
• Python 3 + PySerial
• Linux Mint
• MICROM buzzer (analogue)
• No echo suppression or filtering configured

Question


How can I send a single, clean DTMF 9 tone without it being interpreted multiple times?
Is there a better way to control the duration or avoid false repetitions when using AT+VTS with this type of modem and buzzer system?

Code Snippet


def initialize_modem(ser):
enviar(ser, "ATZ")
enviar(ser, "ATE0")
enviar(ser, "AT+FCLASS=8")
enviar(ser, "ATS0=1")
print("[INFO] Modem ready to receive calls.")

def main():
ser = connect_modem()
initialize_modem(ser)

last_call = None
waiting_for_tone = False

try:
while True:
line = ser.readline().decode(errors='ignore').strip()
if line:
print(f"[MODEM] {line}")

if any(tag in line for tag in ["RING", "R", "NMBR", "NAME", "DATE"]):
last_call = time.time()
waiting_for_tone = True

if waiting_for_tone and last_call and time.time() - last_call > 5:
print("[INFO] Assuming call is active. Sending DTMF tone 9...")
log_event("Call detected. Sending DTMF tone 9")

# Send exactly one tone of 70 ms
enviar(ser, 'AT+VTS={9,7}')
time.sleep(0.5)

enviar(ser, "ATH")
print("[INFO] Call ended.")
log_event("Call hung up.")

initialize_modem(ser)
log_event("Modem reset for next call.")

last_call = None
waiting_for_tone = False
time.sleep(2)
print("[INFO] Waiting for the next call...\n")
except KeyboardInterrupt:
print("\n[INFO] Program interrupted by user.")
finally:
ser.close()
print("[INFO] Serial port closed.")

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