...or you can add 'w' (wait, press * once more) this allows you a little more control - the softkey states "Send DTMF" you can then press it to enter the PIN/telephone number. May be useful if you find it is being sent too early (you could I think add more 'p's as an alternative).
Just set this up for a collegue with 18185.co.uk (0.5p/min to the US...)