@AndrewO It's a Gigaset C610A I am using a SIM, described as "5G EE Unlimited Calls, Texts & Data SIM". I haven't arranged or bought any telephony beyond that. It works fine apart from the echo which...

I have a new NX510V supplied by the TP-Link Store on Amazon , ie. NOT anything custom ISP. I have an EE 5G SIM inserted and my ex-landline base-station connected to the RJ44 port. It works for voice...