Only very old telephone handsets require an external ring capacitor. Anything bought in the last 20 years or so, especially modern DECT phones will work with just a 2 wire connection, with no external ring capacitor required.
Are you sure that you don't have a faulty telephone handset ? I am very surprised that the ATA will dial a number without first giving a dial tone to the connected handset. The dial tone is usually an indication that registration to the SIP server has completed.
If the problem is no incoming ring signal or one way audio then look for NAT issues.
The dect phone I am using is a BT Studio Plus 5500, new and unused for maybe 6 or 7 years but certainly less than 10 years and does not give a dial tone without the ring capacitor in place (which I now have). The other handset tried is an old analogue set 25-30 years old. They both now work with the ring capacitor adaptor.
Just to be clear I was unable to call out as there was no dial tone but I could call the phone and hear a ringing tone but the phone itself never rang.
The phones would both register with the voip provider but apparently showing as being registered is not the same as the phone having a dial tone and being usable.
I have not made any calls yet but at least I am able to ring in and out to phones on the ATA.
Thanks for your input by the way.