If the user enters an MMI string that, according to
TS 22.030, should be treated as USSD, the UE shall send this string to the network using the appropriate operation from
TS 24.080.
The mobile initiated operation shall contain an alphabet indicator and language indicator. The alphabet indicator shall indicate the alphabet used in the operation. The selection of values for these indicators is a matter for the user.
The UE may initiate an USSD operation either during a call or out of call.
A network supporting USSD shall examine the alphabet indicator. If the serving network does not recognize the alphabet indicated in the mobile initiated USSD operation, it shall send the operation to the HLR.
On recognition of the alphabet, the network shall examine the contents of the string, and take appropriate action, according to the following rules, depending of the format of the message.
Case a)
1, 2 or 3 digits from the set (*, #) followed by 1X(Y), where X=any number 0 4, Y=any number 0 9, then, optionally "* followed by any number of any characters", and concluding with # SEND:
This case is reserved for HPLMN use. When a serving network receives such a message from a visiting subscriber, it shall pass the USSD message directly to the HPLMN. If it receives it from a home subscriber, it is up to the network to decide whether to treat it locally or to pass it to the HLR.
Case b)
1, 2 or 3 digits from the set (*, #) followed by 1X(Y), where X=any number 5 9, Y=any number 0 9, then, optionally "* followed by any number of any characters", and concluding with # SEND:
This case is reserved for VPLMN use. It is up to the VPLMN to decide how to treat it.
Case c)
7(Y) SEND, where Y=any number 0 9:
This case is reserved for HPLMN use. When a serving network receives such a message from a visiting subscriber, it shall pass the USSD message directly to the HPLMN. If it receives it from a home subscriber, it is up to the network to decide whether to treat it locally or to pass it to the HLR.
Case d)
All other formats:
The visited network examines the message. If it is able, it acts upon it. Failing that, it passes the message to the HLR.
If the HLR does not support the alphabet indicated, it shall inform the UE.
The network shall terminate the mobile initiated operation by responding to the request from the mobile with either an error signal, or a text string indicating the outcome of the operation. The response string uses the characters available in the selected alphabet as defined in
TS 23.038. If no indication to the user is required, the response string may be empty.
The response to the mobile initiated USSD operation shall contain alphabet and language indicators. The selection of values for these indicators is a matter for the network operator.