What does Windows error code 7015 (ERROR_CTX_MODEM_RESPONSE_BUSY) mean?

ERROR_CTX_MODEM_RESPONSE_BUSY

Busy signal detected at remote site on callback.

ERROR_CTX_MODEM_RESPONSE_BUSY is Win32 error 7015 (0x00001B67) in legacy Remote Desktop Services and Terminal Services session handling.

What to verify

Verify whether the destination line becomes available and accepts the callback call. Check callback number, attempt count and spacing, and modem result code.

Likely causes

• the user line is already in use.
• the callback number points to a busy shared line.
• the PBX returned a busy result.
• repeated automatic callbacks overlap.

Troubleshooting steps

1. Collect callback number.
2. Record attempt count and spacing.
3. Correlate modem result code with TerminalServices operational logs, session and user-configuration records, device or modem diagnostics, and the caller’s connection trace.

Handling, retry, and recovery

Apply bounded retry with sensible spacing, verify the destination number, and avoid parallel callbacks to the same endpoint.

Related errors

ERROR_CTX_MODEM_RESPONSE_VOICE detects a voice endpoint; this code indicates a valid dial attempt that encountered busy signaling

Example

Two session attempts request callback to the same analog line. The second receives busy until the first connection ends, so serialization prevents repeated failures.

References

• Microsoft: Win32 system error range containing this code
• Microsoft: Remote Desktop Services API
• Microsoft: WTS configuration classes
• Microsoft: WTSAPI32 reference