VARCHAR to INTEGER conversion

Hello list

I have a stored procedure that takes in an integer

sp_customer_service_property_descr(INTEGER)

However i have a database field that is a VARCHAR because it can take
either a number that would have to go into this stored procedure or it can
just have normal text depending on the value of another field.

I tried casting the VARCHAR as an INTEGER postgres (V7.2b4) doesn't let me
do this. Now i understand why it is not letting me do this however here is
my question

Does anyone know of a work around so that i can call this function with
the varchar database field.

Thanks in advance

Darren Ferguson