A prefix in SMS Messages are the first two characters in the Sender ID. The first alphabet represent the name of the operator and the second alphabet represent the location.  A Sender ID is the address of the Sender. With the help of a Sender ID we can easily make out from where we are receiving those messages. 
Before we discuss further, let us first see the different types of Sender ID we have:-

 Alphabets Sender ID- It is the message received with alphabets sender ID such as VM-ICICIB, V stands for Vodafone,  M stands for Mumbai and ICICIB means the message is sent from ICICI Bank.

 Numeric Sender ID- It is the message received with Numeric Sender ID such as MN-274657, M stands for MTNL operator, N stands for the location Northeast  and 274657 are the six digit Sender ID.

Below are the lists of operators functioning in India:-

CODE   OPERATOR NAME
A            AIRTEL
B           BSNL
C           DATACOM
D           AIRCEL
E           Reliance Telecom
H           HFCL
I            IDEA
J           JIO
L           LOOP Telecom
M          MTNL
P           Spice Telecom
R           Reliance Comm.
S           S Tel.
T           Tata Teleservice
U           Telenor
V           Vodafone
W          Swan Telecom
Y          Shyam Telecom

Below are the lists of Service Area Codes:-

CODE    LOCATION

A          Andhra Pradesh

B           Bihar
D           Delhi
E           Uttar Pradesh( East)
G          Gujarat
H           Haryana
I             Himachal Pradesh
J            Jammu & Kashmir
K           Kolkata
L            Kerela
M           Mumbai
N           Northeast
O           Orissa
P           Punjab
R           Rajasthan
S           Assam
T           Tamil Nadu( incl. Chennai)
W          Uttar Pradesh(West)
X           Karnataka
Y           Madhya Pradesh
Z           Maharashtra

Now I hope you know where your SMS comes from excluding from the phone numbers  which are saved in your contact list.