Put this in the outbound dial plan of you route.
I believe this is one way to do it:
xx.<#> – Adds a # to the end of any number with 1 or more digits (P98 of the manual). This will dial immediately. Insert it into your own dial plan.
I think this is another way:
S, S0, S1, S2, ...S9 - Digit timer of 0, 1, 2, ...,9 seconds respectively
This can be configured to delay with the seconds you choose
The manual explains better.