Unsupported Analog phone (Gigaset via Grandstream ATA) not ringing on Anonymous calls – no FSK signal

[DT Handel & Consulting]

**Environment:**
- 3CX Version: 20.0
- SIP Trunk Provider: Stadtwerke Ahrensburg (not on 3CX supported provider list)
- ATA Device: Grandstream HT814 V2(converting SIP → analog)
- Endpoint: Gigaset analog cordless phone connected to ATA

**Issue:**
When an Anonymous call comes in through the SIP trunk and enters the call queue, the analog Gigaset phone does **not ring**. However, the phone does show a visual indicator (light/notification on the base station) and the call can be answered normally by picking up the handset. Audio works fine both ways.

If the employee misses the visual cue, the call forwards to the mobile queue after 30 seconds as per the configured scenario.

In the 3CX log, anonymous calls appear as: `anonymous (412862)` — a fixed trunk ID — instead of the actual caller number.

**Root Cause (our analysis):**
Analog phones require an **FSK (Frequency Shift Keying)** signal to trigger the ring. The ATA device sends FSK carrying the Caller ID before the ring signal. When the call is Anonymous, no Caller ID is present → no FSK is generated → the phone does not ring.

This was not an issue previously because the site used a **FritzBox with integrated DECT**, which communicates with the handset via DECT internally — bypassing FSK entirely. After migrating to 3CX with a dedicated Grandstream ATA, the FSK dependency became a problem.

**Questions:**
1. Has anyone else encountered this issue with analog phones via ATA and anonymous calls?
2. Is there a way to configure 3CX or the ATA to send a dummy/generic FSK signal when Caller ID is anonymous, so the phone still rings?
3. Are there any known workarounds on the 3CX side (e.g., manipulating the inbound call headers to replace "anonymous" with a placeholder number before it reaches the ATA)?

Thanks in advance for any input.

 

[AWS2P]

A guess, is there any filter that could be enabled for anonymous calls, excluding the calls considering it might be phone spam?

Personnaly I don't want Anonymous calls disturb me as most of the time this is spam, so i did a rule in 3CX to redirect anonymous calls to an end IVR,broadcasting a joking message .

 

[DT Handel & Consulting]

an inbound rule is indeed a valid approach, and we considered it as well. However, in our case the client (a medical practice) does occasionally receive legitimate anonymous calls from patients who have caller ID suppression enabled on their end, so we can't afford to filter them out entirely.

Regarding the no-ring behavior: our employee can still notice an incoming call through the visual indicator on the phone base, so they are aware a call is coming in — the issue is specifically that there is no audible ring from the handset. For someone away from their desk or not actively watching the phone, the silent incoming call is easy to miss.

 

[paulodagraca]

Hello DT Handel & Consulting,

How about using the "Reformat inbound or outbound caller ID" on the trunk.
Change "Anonymous" => "0000000000"
Would that make the phone ring?

Paulo