Thank you for your help.
I used Linksys SPA 2101. I used ipkall + VoipDiscont.
In Line1, my in/out are with VoipDiscount.
My question is: I can not make CID work. It always shows some random #.. I checked online, many posts said the VOIPDiscont do not provide the CID server. Is that true? Does any one has used VOIPDiscount with successfully displayed your CID? Also I can not find the CID number verification pages from VOIPDiscount.com. Thank you so much for your help.
Updates. I got this CID works. I have a bad cordless phone. After change the phone, it works.