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The message goes to BT because your phone number is still owned and operated by by BT.

If you kept your number when moving from BT to VM your original number will still be listed as a BT number, calls to you get redirected to your VM line by BT as if it was a permanent call forward. Text messages cannot be forwarded, so when sent to you the sender's SMSC will look up the number, see that it belongs to BT, and send it to BT's SMS fudging device.