Make's sense to me. With 3 x DSM32+ (if full, that's 96 sets)
plus 60 IP sets, thats 156 total. If i recall, the more IP sets you add, the more time slots (DSP resources go missing).
If you have added more sets recently to the BCM without perhaps noticing that this new 'side effect' it stands to reason if you 'tead down' some sets and test too see if your voice resources come back, you've isolates the issues.
As Dasheen made mention, the 3/5 split would most likely fix the issues because the split you choose is determined by the number of signaling channels you require for applications such as voice mail, IVR, IP trunks, IP telephones and dialup ISDN WAN connections. If you need 58 signaling channels or less for these applications, use a 2/6 DS30 split. If you need 59 signaling channels or more, use a 3/5 DS30 split.
Based on the 60 IP sets in use.. you need more resources, so the move to 3/5 makes a lot of sense!
You can change from a 2/6 setting to a 3/5 setting without losing data.
Warning: Ensure that the system is idle before you do this procedure. The system needs to be restarted after the setting has been changed. DO A BACKUP!
I'd recommend reading the BCMs context sensitive help:
Contents > Configuring the MSC resources > Changing the DS30 Split
Feature ** RLS: The feature code to fix annoying phone support problems!!