Random no audio problems

System is version Asterisk version 13.9.1
We have a PRI installed with Dahdi version 2.11.1. I have three phones in a Ring Group. Randomly the phones in the group will not get any audio when they attempt to answer a call. I ran tcpdump and could see the system send invites to the phones with the proper codecs, and even when one phone got connected to a call there was no audio. The CDR report logged the call as completed with 13 seconds in length. I was onsite when this happened. Bad Pri channel? This system has about 80 phones on it and these are the only phones that appear to have this issue. This is a school and these phones are in the ADministration office. I would suspect they get the brunt of the calls daily
Ideas? Intermittant bad channel on the PRI or the card? The PRI has been inplace for several years with another system connected. No reports of calls from other phones being a problem