I bought a Sipura SPA-2002 Voip adapter yesterday. It works well with my Voipstunt account.
One of my concern is the bandwidth requirement for conversation.
The FAQ of Sipura said:
Bandwidth usage depends on the audio codec being used. For best voice quality
and FAX, the audio codec should be G711 (by default)
The estimated bandwidth requirement is shown below for 1 channel usage
G711 ~ 64Kbps (can be 90Kbps with overhead), G729 ~ 24Kbps, G723 ~ 17Kbps