that's because you didn't change the tnsnames entry from
PDSOURCE.TCSKOLKATA.CO.IN to PDSOURCE.

"moreover, the box having the uii db has other dbs also which are in use by the team - so it wont be prudent to tamper with that."

your client connection to your server is different with your server/server connection. As long as the listener is up and the tnsnames, sqlnet in client is well configured, then it should just be fine.

Anyway, if you decide not to touch your tnsnames, sqlnet in your server try creating the db link like this(assuming you are creating your db in UI.WORLD):
create database link YOURDBLINK_UII
connect to strmadmin
identified by strmadminpd
using PDSOURCE.TCSKOLKATA.CO.IN