You can activate SMS vouchers, GPRS packs, 3G data RCVs offered by BSNL through SMS. Cost of the voucher will be deducted from your main account balance. 

Here is the list of some of the codes though it is not complete, if you know any other codes please let me know. For details about the stvs/rcvs see the corresponding posts.

GPRS Activation/Deactivation:

Send GPRSPRE to 53773 for activation
Send GPRSD to 53773 for deactivation

GPRS/3G/MMS Settings:

Send <make> <model> to 58355
Ex: Nokia 6300 to 58355

Friends and Family Numbers Addition:

Send FFE <number1> <number2> .... to 53733
Ex: FFE 9491xxxxxx 9492xxxxxx ....  to 53733

Friends and Family Numbers Modification:

Send FFM <old number> <new number> to 53733
Ex: FFM 9491xxxxxx 9492xxxxxx to 53733

Old SMS Special Tariff Vouchers Codes[not working now]:

Send sms13 to 53733 for activating STV13
Send sms23 to 53733 for activating STV23
Send sms33 to 53733 for activating STV33
Send sms52 to 53733 for activating STV52


Send sms23 to 53733 for activating STV23
Send sms52 to 53733 for activating STV52

Voice STVs:

Send VOICE43 to 53733 for activating STV43
Send VOICE59 to 53733 for activating STV59
Send VOICE65 to 53733 for activating STV65
Send VOICE135 to 53733 for activating STV135
Send VOICE160 to 53733 for activating STV160
Send VOICE185 to 53733 for activating STV185
Send VOICE344 to 53733 for activating STV344
Send VOICE399 to 53733 for activating STV399

2G to 3G Migraion:

Send M3G120 to 53733

3G to 2G Migraion:

Send M2G to 53733

GPRS Packs[not working now]:

Send GPRS13 to 53733 for activating GPRS-14
Send GPRS89 to 53733 for activating GPRS-98

3G DATA RCVs[not working now]:

Send DATA21 to 53733 for activating RCV21
Send DATA103 to 53733 for activating RCV103
Send DATA203 to 53733 for activating RCV203

National Do Not Call Registry:

Send START DND to 1909 to activate
Send STOP DND to 1909 to deactivate

Mobile Signal Complaint:

Send SIGNAL to 9400054141

Deactivate Unwanted VAS Service:

Send STOP to 155223 to deactivate

ISD Calling for Prepaid:

Send ACT ISD to 53733 to activate
Send DEACT ISD to 53733 to deactivate 

Mobile TV:

Send MYTV to 55447(not toll free) for 2G customers
Send MY3GTV to 55447(not toll free) for 3G customers

BSNL Caller Tunes:

Send BT ACT to 56700(Rs.2/SMS) for activation
Send BT DACT to 56700(Rs.2/SMS) for Deactivation
Send <song code> to 56700(Rs.2/SMS) for song selection with code
Send <song name> to 56799(toll free) for song search


Send BILL to 53733 for getting the last bill amount
Send AMT to 53733 for getting the usage amount
send email Email-id to 53733 for getting mobile bill by email.