[Serusers] Call between SER proxy servers of different domain

Hi, Was trying to make calls between 2 SIP soft phones which are registered with 2 different SER SIP proxy servers running on different machines but it fails always. When I make a call from Soft Phone-A registered with SER-A to Soft Phone-B registered with SER-B it fails. Soft Phone-A receives 407 response to its INVITE from SER-B and always fails to authenticate. Please provide me inputs on how to solve this problem. I have tried even adding Soft Phone-A's username and password in SER-B but none of them worked. I have another doubt about "aliases" in SER. Can I make some effective use of this and solve the problem mentioned above? Thanks in advance, Ranganath