The Ligo Bluewave is what you want.
https://www.ligo.co.uk/ligo-bluewave-hub
This is a clone of the American XLink, currently being sold for a fraction of the price (around £20). The only catch is its software is old and can't be updated. As you will only be using it with a mobile phone and a vintage dial phone that has no caller display, that shouldn't matter - no software update should be needed.
The good news is the Ligo Bluewave supports rotary dial phones like the 746 straight out of the box. Just plug in the power supply and your vintage wired phone, then pair your mobile phone using Bluetooth. Assuming your 746 has been converted to a modern BT plug, you'll also need a BT to RJ11 adaptor to plug it into the Bluewave. An old ADSL filter can be used as an adaptor, together with the RJ11 cable supplied with the Bluewave.
You can read more about my experience of it here:
https://www.vintage-radio.net/forum/...5&postcount=18
What Joe says is usually true - you can often get a landline + broadband for less than the cost of broadband on its own. But the catch is that outgoing calls from the landline are a lot more expensive than from a mobile. That's why the companies are so keen to offer you a landline - they make a lot of profit on the usage charges. You can take out a package with inclusive calls but that's also extra money.
If you do get a landline in the future you can add it to the Bluewave box. You will be able to make or receive calls on your vintage phone using either landline or mobile. You can choose whichever is cheaper or free / inclusive.