Subscribers rely on USSD for balance checks, top-ups, and mobile money. When they move to VoLTE, the circuit-switched channel that carried those *123# menus is gone. USSI keeps the same dialogs working over the IMS core, so the service does not disappear when the subscriber moves to 4G.
USSD needs no app and no data session and works on every handset, which is why operators built self-care, top-up, and mobile money on it. The transaction runs over the circuit-switched signalling path through the MSC/VLR.
A VoLTE subscriber has no circuit-switched channel. Once a subscriber is on LTE or 5G, the path that carried those *123# menus is gone, and every USSD service built on it goes dark for that subscriber unless it is carried another way.
CSCF receives USSD in SIP and passes it to the USSI function
In IMS, USSD travels as USSI (USSD over IMS, defined in 3GPP TS 24.390). The CSCF delivers the USSD message to the USSI function in SIP. The USSI selects the right HLR, resolves the subscriber's IMSI, and routes the transaction on to the HLR or directly to the USSD Gateway that hosts the service.
USSI resolves the subscriber IMSI via HLR and routes to the USSD Gateway
The Ouroboros USSD Gateway handles USSI alongside legacy USSD, so the same *123# services run unchanged for 2G and 3G subscribers and for VoLTE and VoWiFi subscribers. There is no separate USSD platform for IMS: one gateway serves both.
Self-care, mobile money, VAS — every *123# service reachable for VoLTE subscribers without a second USSD platform.
Operator self-service and account management
Prepaid balance checks, voucher top-up and recharge, data bundle purchase, tariff changes, and last-recharge or expiry queries run as menu-driven sessions. They work on any handset with no app and no data session, now including VoLTE-only subscribers.
Mobile money and financial services
Mobile money wallets — balance, send money, cash-in and cash-out, bill payment, airtime purchase — are delivered through USSD menus because they reach every handset and work without internet. USSI carries the same wallet menus to VoLTE subscribers, with no parallel implementation.
Value-added and information services
News, weather, sports scores, promotional opt-ins, contest entries, and VAS subscription management run as lightweight pull or network-initiated sessions over the same gateway.
Continuity through VoLTE migration
As subscribers move from 2G and 3G to VoLTE, their USSD services keep working, without a circuit-switched fallback kept alive only for USSD.
USSD does not go away when subscribers move to 4G. Self-care, top-up, and mobile money still run through *123# menus, and for many subscribers they are the primary way to interact with the operator. USSI is what keeps those services reachable once the circuit-switched channel is gone.
Ouroboros handles USSI on the same USSD Gateway that already serves 2G and 3G, so operators add VoLTE and VoWiFi coverage without standing up a second USSD platform. The gateway was the first offered as a rental service, and the OPEX model lets the IMS path be added as subscribers migrate rather than ahead of them.
Tell us your IMS core, the USSD services you run, and where VoLTE migration stands. We'll confirm how USSI carries your *123# services over IMS on the same gateway, on OPEX or CAPEX.