Incoming calls from BT and all outgoing calls via VOIP

I am looking for a way to use my BT landline for incoming calls only and VOIP for all outgoing calls ideally using my present equipment.
I have both landline phone and FTTC broadband with BT on a long contract. I also have a Sipgate VOIP account which I use only for calling abroad either using a mobile with Acrobits or by using a dedicated phone plugged into my ATA of which I have two, a Cisco SPA122 and a Gigaset N300.
As I am unable to move the landline number away from BT without terminating the FTTC and having to pay far too much for cancelling the contract I am looking for a way to move away from paying BT for calls out and using my current phone setup to use VOIP for all outgoing calls using the landline caller ID and having all incoming calls via the BT landline both using my current phone setup of a BT DECT base with several handsets registered plus one old analog phone.

I have been unable to find a way to get the Cisco SPA122 to split the calls in and out in the way I want and I don't think the N300 makes this possible using a plugged in phone and the BT DECT phones will not register on the N300.

I know that I can probably use some new Gigaset phones to use for calls out but for domestic peace and harmony I want to use my existing phones. Also the DECT base on the N300 is not as powerful as the existing BT base and does not have a strong enough signal for the entire house, hence another reason to use the existing setup.
I also think it might be possible to use a Draytec router with two phone inputs, the 2766VAC for example but that is an expensive solution.
Is there a simple way of achieving what I want that will work automatically and using my existing kit.

I have a large house with a single N300A installed. My front garden is 50m, side 40m and to the rear even more. Coverage is right out to 50m from the single N300A and if I want more there is a Gigaset extender available.

But Gigaset handsets and sell the Cisco.

You're looking for an ATA with an FXO port to connect to your current landline, an FXS port to plug in a DECT base, and some sort of dial plan support to route calls to different places.

The SPA122 won't do this because it doesn't have an FXO port, and the N300 won't do this because it doesn't have an FXS port, and as far as I know there is no way to have the N300 accept a SIP registration from another device to use its FXO.

On paper the Grandstream HT813 should do what you want but it isn't a DECT base so you will be plugging your BT DECT base into it. It's unclear what is connected to your N300 currently.

In reply to a post by MHC:

I have a large house with a single N300A installed. My front garden is 50m, side 40m and to the rear even more. Coverage is right out to 50m from the single N300A and if I want more there is a Gigaset extender available.

But Gigaset handsets and sell the Cisco.

I have discounted using the N300 as it does not cover my house from end to end well enough to be usable. Whereas my BT DECT base does the job well.
Thanks for your advice nevertheless.

In reply to a post by jpm:

It's unclear what is connected to your N300 currently.

Thats because I only actually plug it in when making overseas calls as these are infrequent. I use a Gigaset C430a base and handset for this. It works OK this way as incoming international calls come in on the BT landline and can be picked up from any other phone in the house.
But I want to move away from this solution, hence my post on the TBB forum

In reply to a post by jpm:

You're looking for an ATA with an FXO port to connect to your current landline, an FXS port to plug in a DECT base, and some sort of dial plan support to route calls to different places.

The SPA122 won't do this because it doesn't have an FXO port, and the N300 won't do this because it doesn't have an FXS port, and as far as I know there is no way to have the N300 accept a SIP registration from another device to use its FXO.

On paper the Grandstream HT813 should do what you want but it isn't a DECT base so you will be plugging your BT DECT base into it.

Well that is encouraging news about the HT813. Will I be able to plug in my landline and next year the digital voice line to one port and the BT DECT base together hopefully with the analog phone to the other port and use any phone for calls out and calls coming in from the BT side will ring on the the other port as well.
This is all far more complex than it needs to be until it is possible to move the phone service away to another supplier without cancelling the VDSL service as well.

Yes but connecting multiple devices to the FXS port will need to be done with a socket doubler or similar.

The datasheet says this can do everything you need it to, but I don't have one to test and haven't configured Grandstream devices before.

In reply to a post by jpm:

Yes but connecting multiple devices to the FXS port will need to be done with a socket doubler or similar.

The datasheet says this can do everything you need it to, but I don't have one to test and haven't configured Grandstream devices before.

@jpm Thanks for your reply, I already use use a socket doubler for the old analog phone and your disclaimer is noted. I will be new to Grandstream too.

I have been using BT (native BT and later through Plusnet) for incoming and VoIP for outgoing for 10+ years. Outgoing VoIP on Freevoipdeal. Massive savings over outgoing calls via BT. Moved to full VoIP (A&A) just over a year ago.

I am using a Fritzbox 7590 (and a 7270 before), both have built in DECT base (and Aswerphone if you need one), so everything is in one box. And you can route some callls (0800, 999 etc,) via BT.

I know you're not considering the N300A because of the signal coverage, but just to say that I've got one (N300A) with a Ligo Extender. This has been working very well for more than 2 years now.
The Ligo Extender is in a warehouse, relaying from the N300A in the office/sales area.
Never had any trouble. I had occasion to check it all out just in the last couple of weeks, as we've opened up a new office in the far end of that warehouse.

Was a doddle to setup.

Just for info.