i betcha will find the trouble is with the incoming calls to the analog trunks and not the PRI.
are you monitoring the inbound SMDR, look at the trunk# on the inbound call. or ask the caller, what # did you dial.
The problem you are having I have never heard of happening on PRI.
Sounds like on the PSTN or on the termination blocks someone has punched down the ring on one line to the tip on another line.