In order to pass it to the next page, you need to add it to the url that points to the next page.
Let us say, your current page containing cellnumber is called "myform.php" and you want to pass it to "myaction.php".
In "myform.php", you put a line with "href" element like this:
<a href="myaction.php?param=value">go to action page</a>
The param is any string you want, used to identify the parameter in the next page ("myaction.php"). So for your example, you put CELLNUMBER value substituting "value".
Now in the "myaction.php" page, you get the value by using:
Of course, if you used other string other than param, simply replaced "param" with the text you used in "myform.php"
For your information, you can also read about it in the PHP Manual.
Note the examples.
I am not familiar with the PHP structure you are using. I assume you are on some sort of framework. What I explained above is a concept, for implementation, you need to check with your framework.