Well, you technically don't need access to callmanager for either.
You can get IP Addresses for phones from the DHCP server, or by simply scanning the voice VLAN and scraping the DeviceInformationX page for device names or phone numbers.
And when you ask, "Post a Get", that just sounds weird, because HTTP support both POST and GET as request methods.
Do you mean to ask: Make a GET request, or Make a POST request? Either way, GET and POST requests, by default are controlled (if at all) by device association. So they do not need to have access to CUCM web pages, jusst associate the device to the user(s).
