If you want to do voluntary work at Mother Teresa's missionary (78 A.J.C. Bose Road, Kolkata – 700016) and ideally would like to walk to work, so here are some information , which might be of some help to you.
First of all the Mothers House & Missionaries of Charities are two different buildings, they are nearby on the same road but little distant apart and metro/unerground/tube train connectivity won't help you.
website : http://www.motherteresa.org/07_family/Volunteering/v_cal.html
-Mr. Gomes house behind Sishu Bhavan.
-Bely Guest House – right next door to Mother house, above the WEB Internet café. There is no need to book rooms in advance. Dorm beds should cost around 100-150 rupees a night.
Sudder St. do have lots of budget hotels, specially for backpackers but it would be little far of, though there are advantages and disadvantages of the place.
PS : Informations are mostly given by a CS Kuntal Das, which I thought is useful.