Glad to hear of your interest in FrontlineSMS. I assume you are asking for advice on which mobile carriers to use and not Internet Service Providers?
I am fairly certain you could use any of the mobile carriers in Uganda to deploy FLSMS. It would be best to choose a company with thorough cellular coverage in the area you wish to deploy.
You might also try going to the Members link on this site and searching for "Uganda" - you will find several individuals working in the country that you could contact regarding local advice.
best of luck and let us know how it goes,