Well, it depends on how good your own hearing is.
But, to some degree, yes, the OBi 10x2 phones will sound better, because the sound isn't being converted from VoIP digital to analog, and then to a telephone line signal, out to the analog telephone and its audio circuitry, and then to your headset, where it gets digitized (over DECT) yet again, and back to analog. Instead, the OBi phone is going directly from the digital VoIP to its own, internal high-quality audio amplifier, which is designed with better components to handle wideband audio (HD Voice). You won't get full HD Voice over standard VoIP calls today, unless you're using a service that supports HD CODECs end-to-end, but the narrowband audio is still crystal clear. The speaker phone is phenomenal on these phones, too. I can't say if you'd notice as much of a difference between the 110 and the IP phone when adding the extra wireless audio path from the Plantronics headsest, but given that it's a modern DECT model, it should still sound very good.
Bottom line: if you are looking for a reason to upgrade, I wouldn't do it just for the audio quality, unless your AT&T phone is causing problems with the headset (distortion caused by impedance or level mismatch). Otherwise, the OBi phone adds some benefits over the 110:
- newer, faster SoC (the computer that runs it), enabling the range of features supported by the OBi 200 and 202 models, such as multiple/simultaneous client registration for Google Voice and Bluetooth and WiFi support
- multiple Google Voice and/or SIP service provider registrations, not just the two on the 110
- the ability to quickly sync and download your Google Contacts to the phone over the internet
- that fantastic speaker phone audio
- the proverbial "and much more", that you can read about on the Obihai website's product sheet for the phones
