Since FM doesn't let you filter related records on the fly, you have to use other methods to get those values:
1. Define a filtered relationship
and create a calculation field or display the related records in a portal
2. Use a filtered portal
3. Define a calculation using ExecuteSQL
4. Use native functions
Here's an example file showcasing all techniques, using a global field as selection tool instead of using a hardcoded value ("cellphone").
Note that #4 can read in all related values recursively, without using a Custom Function (grazie, ChinoguMaker …). Have fun checking it out!
I created a Parent Table of employees with a unique id (userid). Next I created a child table of devices linked by username with a unique id of serial number.
Not sure I understand this part; you should only use serial numbers as relational keys, not any user-definable data (also not in combination with an ID).