1. Get a license from CCK (or whatever it is called nowadays) to provide the service. You need business plan, projections, what you want to do blah blah copy of registration copy of this and that
2. Buy - Lease - Cloud Host the equipment and its software and connect it to the mobile service providers (with letter from CCK at hand)
3. Sell your services to clients. For each short code apply (again) to the mobile service providers with details like what you want to do with it price etc etc
They simply charge an amount above the regular call or SMS cost and split between the mobile service providers, operators and themselves. Or charge fixed fees for hosting services etc etc
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