Hiya,
I've never heard of a Subsidy PIN number before - it really does sound like a 'within the industry' type lock/feature for certain network dependant requirements/features/limitations.
What handset is it ? - I noticed you said 'UK Motorola phone' - so just to clarify, its a UK model, handset from a UK supplier/shop etc ?
I've a feeling it could be a handset which was exclusive to Vodafone at some point, and may or may not have had certain features included/omitted from the same handset but for other networks etc - for instance orange & T mobile do things like that on some of their smartphone/PDA handsets and some of the forums I go to users are frustrated - for instance on a variant on a HTC handset, T mobile helpfully decided to omit the Wi-fi (i think it was) feature, yet on the unbranded HTC handset it was there.
Maybe speak to
1. the place you got the actual handset from
2. the new SIM supplier
3. Look on Motorola forums or them direct at thier technical dept to see if there's any info
Cheers! DtM! West London & Slough UK!