0X00001B66: No Dial Tone Detected in Remote Desktop

1. The Phone Line is Dead or Disconnected

What's actually happening here is the modem tries to go off-hook and listen for a dial tone. If it hears silence or noise instead of the 350 Hz + 440 Hz tone, it throws error 0X00001B66. The real fix is almost always a physical line problem.

I've seen this on Windows Server 2019 and Windows 10 Pro machines where someone plugged the phone cable into the wrong RJ11 jack. Or the cable got crushed under a desk. Or the line itself is dead because the telco cut service. Don't assume the software is broken — check the wire first.

1. Unplug the phone cable from the modem.
2. Plug a regular analog phone into that same cable.
3. Pick up the handset. Do you hear a dial tone? If no, the line is dead. Call your phone company or check the wall jack.
4. If you hear a tone, plug the cable back into the modem. Run hypertrm.exe (HyperTerminal) on Windows 7 or use putty with a serial connection to COM port the modem is on. Type AT and press Enter. If you get 'OK', the modem can talk. Then type ATDT123456 (replace with a real number). If it gives 'NO DIALTONE', the phone line signal is too weak for the modem but works for a handset. Replace the cable or test a different wall jack.

Why step 3 works: A handset amplifies the signal better than most modems. If the line voltage is borderline (below 50V DC on-hook), a phone handset might still work but the modem's tighter threshold won't detect a dial tone.

2. Modem Init String Skips Dial Tone Detection

If the phone line tests fine with a handset but the modem still fails, the modem's AT command string might be set wrong. Many modems have a register S6 (wait time for dial tone) and X options. The default X1 or X3 expects a dial tone. But some routers or RAS configs use X0 which skips dial tone detection entirely. That hides the problem but isn't a fix — it'll dial anyway and fail later.

Example init string to check:
AT&F1 (factory reset)
ATS6=4 (set dial tone wait to 4 seconds, default is 2)
ATX3 (enable dial tone detection, blind dial off)
Then dial: ATDT5551212

To change it in Windows Remote Desktop:

1. Open Routing and Remote Access (rrasmgmt.msc).
2. Find your modem port under Ports.
3. Right-click > Properties > Device tab > Configure.
4. In Extra initialization commands, type ATX3 (no quotes). This forces dial tone check.
5. Apply, restart the Remote Access service.

Don't use X0 here — it'll skip the check but then the modem might dial garbage and timeout with a different error. You want the dial tone check to work, not bypass it.

3. Modem Speaker Volume Too Low to Detect

Here's a weird one: some modems (especially older USRobotics Sportster or Conexant chipsets) have a speaker volume register L that affects the dial tone detection circuit. Setting L0 (speaker off) can actually disable the tone sensor on some hardware. I've seen this on a Dell Optiplex 7050 with a built-in Conexant modem under Windows Server 2016.

1. Open Device Manager, find the modem under 'Modems' or 'Ports (COM & LPT)'.
2. Right-click > Properties > Advanced tab.
3. In Extra initialization commands box, add: ATL2 (medium speaker volume).
4. Click OK, restart the modem via Device Manager (disable then enable).

But note: This only helps if the modem's firmware ties the audio amplifier to the tone detection circuit. On most newer modems (like the ones inside laptops that have no speaker), this won't do anything. If you don't hear clicks or dialing sounds from the modem speaker when it tries, try ATL2 anyway — it's a 5-second test.

Quick-Reference Table

If none of these work, the modem is probably dead or incompatible. Try a different modem or use a VoIP ATA device that simulates a dial tone. Some cheap USB modems have terrible tone detection thresholds — I've thrown away a few Trendnet TFM-560X because they couldn't detect dial tones reliably.