User configurable softphones allow for a lot of flexibility when it comes to operating with various VoIP services of your choice. But along with flexibility comes complexity too.
Everyones PC is different and the reasons you may have problems can be numerous and hard to identify or nail down - and not necessarily the fault of the softphone you are using - it just may not like something about your computer or network configurations. If that's the case, try a different softphone until you find one that seems to like your system configuration best.
From my experience, most VoIP problems are related to NAT Routers, Firewalls, and Codecs. Troubleshooting these issues can be complex and is beyond the scope of this article.
Free User-Configurable SoftPhones