Inbound concurrent calls to Ring Group

I would appreciate any feedback on a strange issue with inbound calls to our ring group. We are using Asterisk and have Sangoma T1 card with a PRI. Essentially the main number rings into a ring group which consists of 90% of the extensions on the pbx. Things normally work fine, except, when two calls come in at what appears to be the same time. As soon as anyone of the extensions answers one of the calls, the 2nd call does not continue to ring the phones in the ring group, and the caller is ultimatey sent to the no-answer voicemail box. I would have assumed that the 2nd call would continue to ring the phones in the group until it to was answered. Any insight would be appreciated.