A complete system for mobile shops that sell phones on installment: an Android app that locks an overdue phone and unlocks it the moment the customer pays, a web panel that runs sales, contracts and collections, and a server that keeps every phone in line.
Kisti Lock is built for Bangladeshi mobile shops that sell phones on easy installments. The shop sells a phone through a four-step sale wizard with a live installment quote, takes the customer's consent on a digital signature, a paper copy or an SMS code, and enrols the phone by scanning one QR code at first start-up. From then on the phone belongs to the shop's lock policy: when an installment goes unpaid past the grace days it shows a full-screen lock with the amount due, the shop's number and an emergency call button, and the moment the shop records the payment it opens again.
The Android app runs as a Device Owner, so the customer cannot uninstall it, factory reset the phone, boot into safe mode or change the date to dodge a lock. Every lock and unlock instruction is a signed document the phone checks itself, so a fake server or a forged SMS cannot open or close a phone, and an offline lease keeps the rules working when the internet is off. When the loan is fully paid the shop releases the phone and the app removes its own control.
The web panel is made for a busy shop counter: a dashboard of today's dues and locked phones, a collection desk with one-tap payment recording and receipts, a fleet view of every enrolled phone with a live preview of the lock screen the customer sees, stock and IMEI control, SMS reminders and templates, reports with Excel export, staff roles and branches. Everything the customer sees, from the lock message to the contract text, is edited from the settings without touching code, and the panel works the same on a phone, a tablet and a laptop in Bangla or English.
Shops that sell phones on installment lose money when a customer stops paying and the phone is out of reach, but a lock that is easy to remove or one that can lock a good customer by mistake is worse than none. The system had to be strong enough to resist a customer who restarts, resets or re-dates the phone, safe enough that nobody outside the shop can lock or unlock a phone, and simple enough for a shop assistant to run from a counter in Bangla without training.
We built the phone side as a Kotlin Device Owner app in lock-task mode that obeys only signed state documents and keeps a time-limited offline lease, and the server side as a Laravel platform where one engine decides every lock from the contract, the dues and the grace rules. Payments, late fees, consent, enrolment and release are handled by dedicated services, the lock screen and contract wording are editable settings, and a virtual phone in the panel lets the team try the whole flow without a real handset.
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