@Silenos Deco doesn't support this feature. To achieve it, you'll need to add an Analogue Telephone Adapter (ATA) to your setup, available on Amazon. Unfortunately, your ISP might not provide the necessary details to configure the ATA, so some research may be required. An alternative could be to cancel your ISP's phone line and switch to a VoIP service. Depending on your country, you may even be able to port your existing phone number to the VoIP provider if you wish to keep it.

Silenos wrote

We have recently upgraded to a Deco Mesh network with two BE85s. We have FTTP but currently I am having to run the network with the ISP's router in pass-through mode connected to the ONT, and then the Deco connected to that. This is because the FritzBox router has a FON socket that we run our VOIP landline phone from.
 

I would prefer to connect the Deco directly to the ONT, but I don't see any obvious way to do so without losing the landline - certainly there is nothing obvious re telephony settings showing in the Deco app. If anyone has any helpful ideas they'd be welcome (or even an official confirmation that it isn't possible).

  @Silenos you can set the Fritzbox to share the current network (see the manual) via port 1 (can't remember if you can specify any port) so that the Fritzbox becomes a switch/AP and can be used as an Analogue Telephone adapter.

Worked well for me when I used it although I disabled the wireless on the Fritzbox to reduce interference.

I think it's a standard firmware feature so it should be available on most Fritzboxes.