We updated to 7.2 SP1 PR1
Load 13.2.1.27
Call control relink 43.2.1.14_SP1
specifically to correct this issue. From what I can gather, so far nobody has reported any more 'no-ring' calls. We had been chasing the issue for around 6 months with no fix from our VAR or Mitel, but it turns out we were running an 'unsupported' patch version, so we updated. The old version was
Release 7.1 PR1
Active 13.1.0.36
CC_relink 43.1.0.32_SP2
HTH,
-dg