Adding to bobcheese:
I always use speedcall tables, even when the DDI matches. In this case I would insert nonsense digits to create my mapping.
Receive digits - 7654321
Trunk Attributes - Absorb 3 digits, Insert digits *#*
Speedcall Table - *#*4321 dials 4321
By using the above method you have full flexibility in translating (or not) the received digits.
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What's most important is that you realise ... There is no spoon.