PowerShell to get values from Azure AD and set them in SharePoint user profiles. Here I am getting all users that are licensed and setting their mobile numbers. This has been run in a production environment and works well. 

Import-Module MSOnline
Clear-Host

# add SharePoint CSOM libraries
Import-Module 'C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll'
Import-Module 'C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll'
Import-Module 'C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.UserProfiles.dll'

# Defaults
$spoAdminUrl = "https://TENANT-admin.sharepoint.com"
$overwriteExistingSPOUPAValue = "True"

# Get credentials of account that is AzureAD Admin and SharePoint Online Admin
$userName = "ADMINEMAIL"
$password = ConvertTo-SecureString "PASSWORD" -AsPlainText -Force
$credential = New-Object -TypeName System.Management.Automation.PSCredential -argumentlist $userName, $password

Try {
    # Connect to AzureAD
    Connect-MsolService -Credential $credential

    # Get credentials for SharePointOnline
    $spoCredentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($credential.GetNetworkCredential().Username, (ConvertTo-SecureString $credential.GetNetworkCredential().Password -AsPlainText -Force))
    $ctx = New-Object Microsoft.SharePoint.Client.ClientContext($spoAdminUrl)
    $ctx.Credentials = $spoCredentials
    $spoPeopleManager = New-Object Microsoft.SharePoint.Client.UserProfiles.PeopleManager($ctx)

    # Get all AzureAD Users
    $AzureADUsers = Get-MsolUser | Where-Object { $_.isLicensed -eq "TRUE" }
    ForEach($AzureADUser in $AzureADUsers){ 
        $mobilePhone = $AzureADUser.MobilePhone
        $targetUPN = $AzureADUser.UserPrincipalName.ToString()
        $targetSPOUserAccount = ("i:0#.f|membership|" + $targetUPN)

        # Check to see if the AzureAD User has a MobilePhone specified
        if (!([string]::IsNullOrEmpty($mobilePhone))) {
            $targetspoUserAccount = ("i:0#.f|membership|" + $AzureADUser.UserPrincipalName.ToString())
            $spoPeopleManager.SetSingleValueProfileProperty($targetspoUserAccount, "CellPhone", $mobilePhone)
            
            Try {
                $ctx.ExecuteQuery()
                Write-Output "User: $targetUPN has had their mobile number set to: $mobilePhone"
            }
            Catch {
                [Exception]
                Write-Output "User: $targetUPN with mobile: $mobilePhone, failed to be set, see exception below"
                Write-Output $_.Exception.Message
            }
        }
        else {
            # AzureAD User MobilePhone is empty, nothing to do here
            Write-Output "AzureAD MobilePhone Property is Null or Empty for $targetUPN)"
        }
    }
}
Catch {
    [Exception]
    Write-Output $_.Exception.Message
}