After logging into your allGeo account,  the Team Page is displayed by default.
You can Select a Phone or a number of phones[1] from the list and then assign them Customer Sites(Job Sites) by clicking on the Mobile Phone Icon[2], which is at the end of the array of icons.
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After clicking on the Mobile Phone Icon, you will be required to confirm the devices you have selected by answering to "Do you want to Assign Customers to all selected Devices?"  alert, with an Okay.

This will open up a new Window which will show you the selected devices[1] and allow you to Assign or un-assign them to and from a customer site.
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To un-assign a device from a Customer, you can select from the list of Assigned Customers and click on Unassign.
In the above image[2], the selected number is assigned to only one customer site named as OFFICE, so only this is available for un-assigning.
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To assign a customer site to a device[3], choose from the list of Customer Name[4] and then click on Assign[5].
After the click on the Assign Button, you will be required to confirm the assignment by answering to " N customers are assigned to all the selected devices.?" alert, with an Okay.
*N would refer to the number of Customer Sites Assigned.