Thank you so much.
Yeah, I also don't believe PC 100% stable like hard phone although I am a developer myself.
Seems only this will cause issue. As my experience I tried ekiga, twinkle in linux, quite not stable. For windows haven't try should be better.
As the key factor won't be impact latency and voice quality. It's the same.
For personal use , thinking no need to purchase a hard phone any more. Of course for business using purpose get a hard phone is better like you said some important call need to be stable when online.
I will use soft phone ^_^ to saving costs. Thank you again.