Handling CIMD negative submit responses ?

The CIMD driver src and the Nokia spec. all suggest that the
CIMD sequence number is incremented in the case of a negative
submission response from the SMSC.

However, I'm testing with an operator where a negative
submission response (prepay credit failure) does not 
appear to increment the sequence number on the operator side so
that the next PDU sent from the CIMD driver fails with an
invalid sequence number error. If I reuse the sequence number,
its fine.

Anyone seen this before ? The prepay checker systems are usually
not supplied by the SMSC vendor and sit as a proxy between the
ESMEs and the SMSC.