Fix ERROR_CTX_INVALID_MODEMNAME (0X00001B62)

Wait, your modem name is wrong

You're getting 0X00001B62 — ERROR_CTX_INVALID_MODEMNAME — and it's not your fault. This happens when Remote Desktop (or any Remote Access Service client) tries to dial out using a modem, but the modem name you entered doesn't match what's actually registered in Windows. The OS is saying, “I don't know that modem.” Let's fix it.

The fix: edit the modem name in the registry

Press Win + R, type regedit, hit Enter.
Navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\RAS\AutoDial\ModemName
You'll see a string value called ModemName. Double-click it.
Replace whatever's in there with the exact modem name from your phonebook entry. The phonebook is usually at:
C:\Users\%USERNAME%\AppData\Roaming\Microsoft\Network\Connections\Pbk\rasphone.pbk
Open that .pbk file in Notepad. Look for a line starting with ModemName=. Copy that name exactly, including underscores and spaces.
Paste it into the registry value.
Close regedit, reboot, try again.

Why this works

What's actually happening here is a mismatch between two places Windows stores modem names. The RAS (Remote Access Service) subsystem reads the modem name from the registry key above when initializing a dial-up or VPN connection. But the phonebook entry — the actual connection profile — might have a slightly different name. Maybe you installed a new modem driver and the INF file changed the friendly name from “Standard 56000 bps Modem” to “Standard Modem”. Or you renamed the connection in the GUI, which doesn't update the registry entry. The reason step 3 works is that you're forcing the RAS service to use the name the phonebook expects, instead of whatever stale name is sitting in the registry.

Another common trigger: you upgraded Windows from 10 to 11 and the old modem driver got replaced. The registry key holds the old name, the phonebook holds the new one. Boom — error 0X00001B62.

When the registry key doesn't exist

If the ModemName key isn't there at all, you need to create it. Right-click the AutoDial folder, select New > String Value, name it ModemName, and set it to the name from your .pbk file.

Less common variations of the same issue

1. TAPI device enumeration mismatch

Sometimes the modem name is correct in both places, but the Telephony API (TAPI) can't find the device because the modem class GUID changed. Open Device Manager, find your modem under “Modems”, right-click > Properties > Details tab > “Class Guid”. Should be {4d36e96d-e325-11ce-bfc1-08002be10318}. If it's different, the modem isn't properly registered as a modem — reinstall the driver.

2. RASMAN service not running

Go to Services.msc, find “Remote Access Connection Manager”. If it's stopped, start it and set it to Automatic. Without this service, RAS can't read the registry key at all, and you'll get a slightly different error (0X00001B64), but I've seen 0X00001B62 in this scenario too.

3. Corrupted phonebook file

If the .pbk file has a blank or malformed ModemName= line, Windows throws the error. Open the .pbk file, delete the whole connection entry (it's bracketed by [Connection Name] headers), and recreate the connection from the Network and Sharing Center. Then re-set the registry key.

Prevention

Don't rename modems in the GUI after you've created dial-up connections — the registry key doesn't update. If you must change the modem name, update the registry key before you dial. Also, after installing a new modem driver, always open the .pbk file and verify the ModemName= line matches what's in Device Manager under the modem's “Friendly Name” property.

One more thing: if you're not actually using a modem — maybe you're on a VPN or PPPoE connection — the error can still pop up if the RAS subsystem thinks it needs a modem for that connection. In that case, delete the modem entry from the .pbk file entirely and recreate the connection as a “Broadband (PPPoE)” or “VPN” type, not “Dial-up”.

This is one of those errors where the fix is simple once you know where Windows hides the mismatch. Now you know.