$OU = ''
$userpool = Get-ADUser -Filter * -SearchBase $OU -Properties mobile | Select-Object SamAccountName, Mobile

foreach ($user in $userpool)
  {
    if ($user.mobile[0] -eq '8')
      {
        $user.mobile = '+7' + $user.mobile.Substring(1,$user.mobile.Length - 1)
      }
    if ($user.SamAccountName.Length -ge 2)
      {
        $user.mobile = $user.mobile -replace ('[^\w+]','')
        $goodNumber = $user.mobile.Insert(2," ").Insert(6, " ").Insert(10, " ")
        Set-ADUser $user.SamAccountName -Replace @{mobile=$goodNumber}
      }
  }