You can make cheap calls from Linksys PAP2 v2 SIP device with SuperJUMBLO. This blog article  detailedsetting up guide for this device.
Connect the power, an Ethernet cable to your router and a phone to Line 1. Find out the adapter’s IP address from your router’s DHCP list, then open a web browser (Internet Explorer) and type in that address. The default username is admin and the password is admin.

PAP2v2_1

Click Voice from the top menu to get into the VoIP settings section.

PAP2v2_2

Now click Admin Login on the right then Switch to Advanced view in the middle, to get access to Advanced VoIP settings pages.
PAP2v2_3

Now you’re ready to configure the VoIP settings. First, we’ll adjust some of the SIP parameters, so click the SIP submenu.

4

Change the following parameters:
(in the middle of the page)
- RTP Packet Size: 0.020
(at the bottom of the page):
- Substitute VIA Addr: Yes
- STUN Enable: Yes
- STUN Server: stunserver.org

Now click the Line 1 submenu

5

Enter the following settings:
(at the top of the page)
- NAT Mapping Enable: yes
- NAT Keep Alive Enable: yes
(about half way down on the page)
- Proxy: sip.superjumblo.com
- Register: no
- Make calls without reg: yes
- Display Name: enter your name here
- User ID: this is the username that you created on SuperJUMBLO website
- Password: the password you set for your account
- Dial Plan: use the following string (including parentheses)
(*xx|<:001>[2-9]xxxxxxxxx|<:00>1[2-9]xxxxxxxxx|<011:00>xx.|00xx.)

That is all, click Save Settings at the bottom to save all the changes. The adapter will reboot and after 2-3 minutes you should get dial tone and should be able to place and receive calls.

Notes:
Normally, SuperJUMBLO requires the numbers to be dialed as 00+Country code+Number (this is the international dialing format from Europe). By setting the dial plan as above, you will be able to dial in this format as well as the North American format (011+Country code+Number). In addition, North American numbers can be dialed directly as Area code+7 digits or 1+Area code+7 digits.

Courtesy –    www.voipfan.net