Probably easiest to build the string first:
Dim strURL as String
strURL = "http://xxxxxxx.com/excelAPI.php?customer_id=1&mobilenumber=" _
& ActiveCell.Value & "&message=" & ActiveCell.Offset(0,1).Value
Call Sheets("Sheet1").WebBrowser4.Navigate(strURL)
Assuming the active cell contains the mobile number and the cell to it's immediate right contains the required message, otherwise specify the cells:
Dim strURL as String
strURL = "http://xxxxxxx.com/excelAPI.php?customer_id=1&mobilenumber=" _
& Range("A1").Value & "&message=" & Range("B1").Value
Call Sheets("Sheet1").WebBrowser4.Navigate(strURL)
You may need to qualify your range worksheets.
EDIT
As requested in comments to cycle through selected cells:
Dim cell As Range, Rng As Range
Dim strURL as String
Set Rng = Selection
For Each cell In Rng
strURL = "http://xxxxxxx.com/excelAPI.php?customer_id=1&mobilenumber=" _
& cell.Value & "&message=" & cell.Offset(0,1).Value
Call Sheets("Sheet1").WebBrowser4.Navigate(strURL)
Next cell
Set Rng = Nothing
Only select the cells that contain the mobile numbers, otherwise the code will try to send to the messages as well. You may want to write in some check to ensure the cell contains a number such as:
If IsNumeric(cell.Value) Then
Or a more detailed format check depending on what you have in the columns of the worksheet.