JoinFieldValue from Account to Lead

 Hi All, 

I have searched the boards for this question and have come across some answers but the workflow still does not seem to be working for me. What I am trying to do is if there is no Account Phone # on the Lead detail when new record is saved, have the Account Main Phone pull onto the Lead and fill in that field. 

This is what I am doing for my workflow:

Record Type: Lead

Trigger Event: When new record saved

Actions:

Wait: Today()

Update Value:

Field: Account Phone Number

Value: JoinFieldValue('<Account>',[<AccountId>],'<MainPhone>')