It is not possible to sync the project milestones and meetings with the calendar in Zoho CRM.
The tasks, meeting and milestones assigned to you across all the project can be seen in the Google Calendar with the help of the gCal link available in the My Work Calendar section.
This copies all your project events from Zoho Projects to Google Calendar.
If you are the Portal Owner, you will be able to create invoice for the billable timesheet entries from Zoho Invoice.
Contacts can be imported from CRM into Projects once these services are integrated with each other. To integrate these two services, refer the link Import contacts from Zoho CRM
Zoho Projects and CRM can be integrated with each other. Refer the link mentioned below to integrate these two services.
If you don't access your configured Dropbox account for more than 15 days then sync process will stop.