IP Phone Problem

Dear helper,

My station behaves strangely while running a softphone,

I have a softphone on each station in the company’s network, A and B,
Both “ping” eachother well,

I can make a PC2PC SIP Call from A to B, and the B rings, So far so good;
But whenever I try calling A by B, nothing happens,
in the B, the softphone shows, Creating outgoing call, and it keeps showing it,
in the A, nothing happens,

I wonder what it can be, any comment/hint would help,
I have tried other B stations, same senario,

I think this relates to this forum because This A station acts the same way with the Asterisk,
Bythe way, the softphones I use are, Sjphone and both have the Same settings,
Any help is appreciated in advance,
Regards,
Saeed