Dialling a number on a SIP trunk would result in a wait of 10 seconds from the last number key press until the Aspire sent the SIP INVITE packet. This was really annoying and not really acceptable as the same delay did not seem to occur on an ISDN trunk.
As a workaround, if a # was entered after the last digit it forced an immediate dial and bypassed the wait timer.
A more permanent solution was to adjust programming option 21-01-03 External Call Inter-digit Time to 5 from 10. This halves the time out until dialling occurs but set any lower could cause issues with users not being able to dial digits fast enough.