Please let us know your exact problems for upgrade from V3.1 to V5 with your Mediant 1000 PRI gateway. Actually, I'm also hit few issues for this upgrade. My gateway (Patton SN4960 PRI T1) also works fine with V3.1. However, it cannot fully works with V5 by default setting. Such as it unable to accepted incoming calls, unable to send caller id to Telco and etc. At that period, we use so many times overnight to try to find out what's wrong at the setting. But no luck. OK. We give up to use V5. Then downgrade 3CX to V3.1, all the things are back to normal.
But V3.1's features not fully satisfy most of users. Most end users are request BLF function. V3.1 can? NO, it cannot. Lucky, 3CX support team send us the new template for generate the new gateway config file for V5. It works except the caller id for send to Telco. Since it is not the 3CX issue. We fixed it by work with Telco's support.
We are now can success to use one gateway for two different version of 3CX Phone system V3.1 and V5.