[Serusers] How to use a numeric ID to call another UA at another server?

Dear ALL: My scenario is two domains(domainA, domainB), two SER sip server (SER_A, SER_B) and two UAs (UA_A, UA_B). If UA_A registers to SER_A using a numberic ID (886701234567) and another UA_B registers to SER_B using a numberic ID (886207654321). How can SER_A know to forward to call to UA_B under SER_B with only numberic ID? Is it possible? If it is impossible, now SER_A and SER_B using the same DB, does this question have answer? How can SER_A search the DB and find the UA_B's URI or 886207654321(a)domainB.com with only 886207654321? Can someone give me a ser.cfg to reference? Please.... Charles