Noted , you cannot have a PHONE Extension PORT in the Target room.
No. not with sl2_141 and WA3002_g4 wifi routers ( Both same ).
Option 1 :
You can try a high power Buffalo WIFI router with WDS facility.
http://www.buffalotech.com/files/pro...r-g300n_DS.pdf Option 2
:Try a Dish Type antenna with either WIFI router.
( Constructed or commercial )
Para two :
You had not mentioned the height involved between
computer room ( with BSNL Phone PORT)& : Target room.
20 to 25 meters Vertical height ?
TEST
If you have a Cordless PhoneBase ( 2.4 GHz model preferred ) at Computer Room, and can
page the Handset at (Target Room )
Or use Intercom:
option 1 with single unit MAY work !
ADVANCED "GYAN": Universal Wireless Repeater - DD-WRT Wiki