OK Mike,
Just to clarify... shorting the mic reduces the crackling but doesn't improve the dialling-out.
But do you mean that fitting the No.21 dial from the 711 (in place of the No. 54) resolves the dialling-out problem, or not?
Nick.
P.S. the Dial No. 54 was a cheapie plastic dial popular from the late 1970s at a guess. Like quartz clock movements they're great until they go wrong, but they weren't really designed to be repaired, just replaced when they failed. Some of their components are now rather fail and prone to cracking:
https://www.britishtelephones.com/dial1.htm#d54