ADSL LINKSYS-ROUTER ASTERISK(maybe spesific SIP)

Hi all,

i just finished the instalation/config of asterisk to my new pc!
Everything seems to works fine! (note: I am using www.voipbuster.com as provider)

when i call a cell phone everythink goes ok my cell is ringing and i can -----answer--------
but when i call a landline the phone is ringing normaly but when i pick up the handset my router looses everything (connection,Sync etc)!

can anyone tell why this is heppenig and what can i do about it???

my network is 1)Linksys gateway router modem(adsl)
Pentium 3- 550 (asterisk’s srver)
3)my main pc a p4 @ 3Ghz (used as client and has install the softphone)

Thanks in advance
Ap

Ps
Test 1
i tried to connect with my soft phone to voipbuster directly and i get the same complete loss

Test 2
i register a new acccount to a sipprovaider and then i ried to connect using the softphone only but here comes the pain thr samr ptob ) cmplete loss