Using OpenWRT with bridged ONT while keeping ISP SIP (RJ11 → RJ45 phone)

Installing and Using OpenWrt
1

Title: Using OpenWRT with bridged ONT while keeping ISP SIP (RJ11 → RJ45 phone)

Hi everyone,

My ISP provides a fiber ONT/router combo that includes built-in VoIP service through an RJ11 phone port. The VoIP works fine when using the ONT as a router.

What I want to do is:

  • Put the ONT into bridge mode
  • Use my own OpenWRT router (mainly for SQM / latency control)
  • Still keep my landline VoIP service working, but instead of RJ11 on the ONT, use it through my own setup (RJ45 / IP-based)

I was able to extract the SIP credentials from the ONT (username, password, server, etc.). However, when I tried to test them using MicroSIP on Windows, I couldn’t get it to register or work properly.

So I have a few questions:

  1. Does OpenWRT support SIP/VoIP clients natively?
    (Or would I need something like Asterisk, FreeSWITCH, or an external ATA?)

  2. If the SIP credentials don’t work in MicroSIP,
    is there any chance they would still work on OpenWRT?
    Or does failure in MicroSIP usually mean the ISP is restricting usage (VLAN, IP binding, etc.)?

  3. Has anyone successfully:

    • Bridged their ONT
    • Used OpenWRT for internet
    • AND kept ISP-provided VoIP working outside the original RJ11 port?

  4. If direct SIP doesn’t work, what’s the best workaround?

    • Keeping ONT only for VoIP?
    • Using an ATA device?
    • VLAN setup for voice traffic?

I’m trying to get the benefits of SQM (for gaming/latency) without losing the phone service.

Any insights or similar experiences would be really helpful.

Thanks!

2

Please avoid using ai here, it gets really annoying.

good idea.

possible/ recommended.

generally possible, but depends heavily on the details (of your SIP ISP and your local 'phone hardware').

While you can run asterisk/ freepbx on OpenWrt, I would advise against that. While possible, this is pretty complex to set up - and even more so to configure in a secure way.
Do yourself a favour and use a dedicated SIP pbx (AVM Fritz!Box? depending on where you are located), SIP ATA (Cisco, Grandstream) or SIP phones (e.g. Grandstream, Yealink, gigaset SIP-DECT etc.) instead. Depends a lot on your region/ country, what expectations you have and what kind of phones (or other phone machinery, fax, etc.) you need - as well as the given in-house cabling.

Not enough information to guess.

yes

yes

Didn't even try, just bought my own hardware instead (the ISP provides a mere ONT, if you tick the boxes accordingly).

Possible, wouldn't be my choice though.

Yes'ish, but don't marry yourself to the term ATA - there are more options. Wired desk phones can be replaced with quite affordable (nice/ better) SIP phones (Grandstream or Yealink should be relatively easy) - if the in-house cabling allows that. DECT phones with cat-iq 2.x/ gap support may be paired to an SIP-DECT base station (e.g. the professional gigaset or yealink model range, not the consumer models). If you need a mixture of all of the above, I've found that an entry level AVM Fritz!Box with current OEM Fritz!OS versions can provide a decent mixture (10 SIP accounts, external and internal, 6 DECT cat-iq 2.x handsets, 1-2 rj11 analogue FXS ports, internal ISDN S0 bus on older models; used exclusively for its phone capabilities, behind the OpenWrt in a segregated subnet, not using its modem, routing/ firewalling, switch, wlan features) of that. Details depend a lot on your needs/ wants.

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Check the original configuration on the ISP device, and tell us which ISP and country. Sometimes you need specific a configuration to reach the SIP servers.