Job timing and recording on mobile device that links back to a CommitCRM database

We are a small IT firm and use CommitCRM as our job management and invoicing system. We need a Windows Phone application written that can interface with this system. We will use the program to keep track of how long we spend on site at each job and it will automatically record the charge in CommitCRM for us. It needs to be able to work without a mobile internet connection as we often see clients in rural areas without coverage. When the program initially loads, it will need to know which technician is doing the job (this can be a setting on the phone as it doesn't change very often), have a button to start timing (this can also pause timing as well) and a finished button which is pushed when we have completed the work. The front screen should show us how long we have spent on the job as a timer. Once the finished button is clicked we need to select the client from a list and then possibly a job (often there is no job assigned so there needs to be an option to continue without selecting a job). We can then alter the hour rate and total time spent (works in quarter of an hour increments). We would then need to specify what was done on the job and if the job is now completed (this marks it as completed in CommitCRM) and finally record the charge. It is important that if the phone is switched off or the application is closed that the timer still continues. We need to either cancel the timer or record that we have finished. CommitCRM does have an API which can be exposed as a web service. There are more details available on their site.