01 What USSD actually is
USSD — Unstructured Supplementary Service Data — is a channel inside the GSM signalling system itself, older than the smartphone by two decades. Dial a star-hash code, your handset opens a real-time session with the operator's computer, and a reply pops up within seconds. No data connection, no app store, no login: it works identically on a flagship and a Rs 2,000 feature phone.
That simplicity is why it survives. Sessions are transient — nothing installs, nothing caches — yet they carry the highest-frequency queries in mobile life: balance, validity, own number, data packs. In network terms, checking your balance by app means waking up the internet; doing it by USSD means whispering down a line that never hangs up.