What’s needed to create a internal home telephone system

I’m looking for some advice :)

I want to have the ability to pick up a home phone in the house that will connect to a phone in a garden room and use to call out to it.

The phones are in with RJ11 ran out and currently connected to a router for external calls.

I intend to cancel the home phone package element of my broadband as I know no longer need it. It worked when I picked up the receiver to speak to the garden room but it beeps and gives normal phone tones.

I am wondering what kind of PBX do I need to set up a new system without beeping etc..? I don’t need to make external calls just ability to use as an internal phone system.

Would it be something like this?

https://www.amazon.co.uk/dp/B0816RSJ3S/ref=sspa_mw_detail_1?ie=UTF8&psc=1&sp_csd=d2lkZ2V0TmFtZT1zcF9waG9uZV9kZXRhaWwp13NParams#aw-udpv3-customer-reviews_feature_div