Found the solution to this here.
Adapt Dial_Plan to include 'P30' which sets the off-hook dial timer.
Before:
S:1, ([#*][*x]x.|19xx|10[1-2]xx|0[1-9]xxxxxxxx|S3 [#*x][*x][*x][*x][*x][*x][*x][*x][*x][*x][*x].)
After:
S:1, (P30|[#*][*x]x.|19xx|10[1-2]xx|0[1-9]xxxxxxxx|S3 [#*x][*x][*x][*x][*x][*x][*x][*x][*x][*x][*x].)
Also need to adjust the 'Dial_Tone' to avoid it changing the sound of the dial tone after 30 seconds:
Before:
350@-19,440@-19;10(*/0/1+2)
After:
350@-19,440@-19;30(*/0/1+2)