IPKall on some of its phone numbers is very bad quality. I've used the service before and the number I got assigned and started to publish ended up having all the problems you are describing, cut-off words, random noises. Beyond being generally unuseable by cutting out words every 20 seconds, it sometimes started garbling everything for a minute at a time.
It is possible deleting your current IPKall number and requesting a new one will solve the issue, though it wouldn't surprise me if it didn't.
I think they support u-law and G729, so toggling the preferred codec in your SIP device might help but that is hard to see as well in my case, where my uplink bandwidth vastly exceeds the requirements for multiple simultaneous u-law calls so there was no reason to go with G729a other than to see if it made IPKall behave better.
Ultimately I abandoned IPKall and got a number with FreeDigits (
http://www.freedigits.com/ ), which has zero quality problems and can be configured to forward to a SIP URI just like IPKall--so I just forward my number to my Voxalot account in the FreeDigits account portal.