7200S v4.70
PRI from Integra

Customer A) Phone rings with Caller ID, no one there. Phone rings right back again with same Caller ID, customer is now on.

They say this happens a lot but I haven't been able to catch it.

Phone rings with Caller ID and they answer it and no one's there. The phone rings back again with the same Caller ID and when they answer it the caller is on the phone this second time.

System installed for six months, they've had the issue since the beginning.

Integra has them on a soft-switch and says they can't switch them to a DMS or anything. Integra is a pretty mainstream carrier.