Well, normally you would call her number, but she exchanged numbers with him, so you would call his number, except that his number is now her number and her number is now his number so you would actually call her number which was his number but is now her number because if you called her number that was her number it'd go to him. In conclusion, you would call her number.
But they could each setup call forwarding to each other's exchanged phone numbers so they would receive the right phone calls.
Edit: No, wait, hold on. That sounds like I just created an infinite loop.
Call to A gets forwarded to B which is set to forward it to A which will forward it to B which then forwards it to A...
I only have 2 phone numbers, so I can't test it.