I'd like to know if I can update fields on achild object with the process builder in following case:
1. my master object = "contact"
2. my child object= "relationship"
3. principal detail field on "relationship" object= "contact name"
4. lookup field on "relationship object"= "in relationship with"
5.phone number related to value showing up in "in relationship with" is already updated by a process when record is created

concretely: 
to add contact A as a relationship to contact B, I populate the field "in relationship with" in my "relationship" object with the name of contact A. When I save, the phone number field reflects contact A's phone by process.

Now, If I change on contact tab the phone number of contact A, I would like it to be also updated also in my relationship object.
I created the follwing process for that but it does not work. Any suggestion is welcome

1. start from "contat" object when record is created or updated
2. following formula evaluates to true: ISCHANGED([Contact].Phone )
3. update records on "relationship" object upon following condition: field "in relationship with" equals reference [Contact].ID
4. Field "phone number" in relationship object to be updated with value in field phone on contact tab: phone number reference [Contact].Phone

thanks!