How to populate Contact info on the Account record

I need to pull in the primary contact Cell # (on the Contact record) into a field listed on the Account record.  I have created a new field called Primary Contact cell# under the Account layout, and have created a joinfieldvalue expression to default this in.  I  have created a BMRS workflow to try to update the new cell phone field when the primary contact is updated and I have the following expression on the WRC IfNull(PRE('<PrimaryContactFullName>'),'Invalid')<>IfNull([<PrimaryContactFullName>],'Invalid').  I then have the following field update JoinFieldValue('<Contact>',JoinFieldValue('<Account>', [<AccountId>],'<PrimaryContactId>'),'<CellularPhone>').  This works fine when the primary contact is being added for the first time, but does