PCMU (G711u) is used by default. Both PCMU and PCMA will give you toll quality but their bandwidth consumption is also the highest. If your network bandwidth is low, you can choose a lower-bit-rate codec such as G723 or G729 which will give you near toll quality at much smaller bandwidth consumption. If bandwidth is not a concern try using PCMU or PCMA, or even the wideband codec G722 which will provide hi-fidelity voice.
For example: If you have a basic high-speed connection at home (768kbps/128kbps), configure your phone with either G723 or G729 to ensure best available voice quality. If you have T1 bandwidth capacity, configure your phone with either G711 (a/µ) or G722 to experience toll-quality or high-fidelity voice.
All the phones support similar codecs and are listed below. The BT100 does not support GSM and the GXV does not support G722. Each codec has its uniqueness for certain application.HT Series - PCMU (G711u), PCMA (G711a), G729A/B, G723.1, G726-32, iLBC