@jnaugle also worth noting, no one actually delivers PRI today, that entire market is a scam. It's actually SIP and then they translate the SIP to PRI on your network edge to make it "sound cool" for people who are out of date and remember when PRI was both still available and actually practical (early 1990s and earlier.) PRI is not nearly as powerful as SIP, nor as cheap, so phone providers long ago "all" moved to providing SIP instead (PRI isn't possible without a physical T1) and "virtualizing" PRI on top of it. All the PRI does it increase cost and lower performance, flexibility and reliability.

So the best solution is to get the phone provider to stop pretending to do PRI and just give you the raw SIP. This improves your phone quality by removing unnecessary steps and limitations, removes the need for you to convert back to SIP (gateway, card, however you do it) and makes everything better for everyone. The PRI interface has no advantages, only disadvantages. It takes the better SIP, and cripples it.

For people running ancient phone systems from before VoIP, SIP isn't possible, so these terrible PRI interfaces are useful for them to continue to be able to hook up those ancient phone systems while still being able to connect to current phone companies. They aren't totally useless, but they are useful only for that one terrible niche case.