So, will I have to switch ISP, as it appears all other ISPs offer a landline service via FTTC, by plugging phone into router.
Not "all other ISPs": getting voice from the same provider will limit your choice of ISP. For example, PlusNet don't do digital voice.
The main ones I know that supply a digital voice service are BT, EE, Sky, Vodafone and Zen (you pay extra for it on the latter).
As others have suggested, separating the voice service to a different provider will give you flexibility to change provider, for example if an altnet becomes available in your area. And it also means you can switch provider without transferring the voice number, which is usually OK but does come with some risk.
Of course, you need to pay extra for the VOIP service. If you make few outgoing calls, then the AAISP PAYG service at less than £2 per month is the best deal around.
If you make lots of outgoing calls then there are many VOIP providers who will sell you a bundle with inclusive minutes in the £8-£15 per month range. However, you're more likely to be better off making calls from your mobile phone, where you can get 1000 minutes or unlimited calls for £5-£10 per month.