Web based Appointment booking system

Web based appointment booking application My web site need a web based appointment system developed using PHP and mysql Brief of requirements A from USA, B from Bangladesh, C from china ,D from Denmark are customer support assistants (CSA) working in my site. They start work from their local times and have a flexibility to start and end work (freelancers). They must be able to delete, enter, edit from the back ends All support workers have their own profile page. In that profile page their weekly working scheduled must be able to display. As a example, if someone visit A’s profile he/she can see A’s working schedule (availability) CSA has set times for a jobs example 20 minutes or 10 minutes. Each set time call a slot, which slot they may work on based on their seniority. So CSA would be able to set his working slot from back end where visitors can see it either they work on 20 minutes or 10 minutes. E from England, F from Finland, G from Ghana, H from Hong Kong are clients come to take the service from our CSA, they come and check our CSA profiles. Then these clients must be able to see our CSA ‘s working times in clients local times and dates. So the clients can create a appointment. example if E preferred A and E can see A’s working schedule in England time, so E book A for a number of slots that E want to have the service from A ( 2 slots could be 2*20 minutes or 2*10 minutes) Once E confirm the slots it will take to a check out and payment will be process Once the payment has completed, there will be a confirmation SMS sent from the system to CSA’s phone. This SMS facility should be covered worldwide as CSA are coming from different part of the world. That’s the appointment system I need pretty much, open to your suggestions as well...