Choppy sound/connection indicates that you don't have enough upstream bandwidth to support your VoiP conversations.
Most services use the G.711 CODEC, which requires 80kbps in each direction for each conversation. If you only have, for example, 128kbps upstream and also have a game player, FTP session or some other function going on in your home while trying to have a phone conversation you could end up with your VOIP phone not being able to get the bandwidth it needs to support the voice path.
You could also check with your VOIP provider to see if they support any of the lower bandwidth codecs like G729/G723.
Note: Make sure all the codecs are enabled under Audio Configuration under line 1.
Then, make a call by using the force codec selection * code, for example, the default for forcing the G729 codec is *02729, you can get all the codes under "Outbound Call Codec Selection Codes" Under the regional tab. Go through the entire list, the ones your voip provider supports will result in a successful call, others will cause invalid/reorder. Play with the ones that work and see which gives you the best sound quality.