if modem is provided by MTNL then just take it to exchange when submitting surrender letter. because once when i had gone to exchange i overheard conversation of mtnl staff and person:"modem MTNL ka hain na to woh pehle leke aao"make sure that u write in letter that, "i am returning modem provided by MTNL with this letter.", because otherwise u wont get proof that u returned the modem.