Thanks for your useful code. I found it necessary to add a pause after each tone for effective dialling in UK

There must have been an inter-digit interval in which there is no energy detected at any of the DTMF frequencies. For compatibility with ANSI T1.401-1988, the minimum inter-digit interval shall be 45msec, the minimum pulse duration shall be 50msec, and the minimum duty cycle shall be 100msec.

(From http://nemesis.lonestar.org/reference/telecom/signaling/dtmf.html)