Assuming you have a decent Internet connection, the best you can do is prioritise the less lossy codecs ahead of the more lossy ones. Check out MOS (medium opinion scores) for codecs under normal conditions and under packet loss if you want some more background. You really only need to have G.711u (aka PCMA), G.711a (aka PCMU) and G.729 enabled as the others are typically only used in calls that stay on the providers network if they are used at all. That is also the order from highest to lowest priority you should use in the NANP area with G.711a ahead of G.711u for the rest of the world. You might see if you could also get away without G.729 if you have poor voice quality on incoming calls but it’s possible the lack of that could be a problem in some cases so you should test extensively.
Really, the best the OBI can do and for that matter many providers is to match the quality of a decent landline aka toll quality. It’s possible your DECT phone is lowering voice quality depending on what codecs it supports. So you should compare with a fixed line phone or a SIP softphone with the same provider. There are higher fidelity codecs than G.711 such as G.722 but around my part of the world upstream provider support is pretty limited but you’ll need other gear for that and potentially another provider.