An i5 and 16 gb ram is a bit much, we have Atom E3845 Quad Core boxes with 4GB ram running 100 phones in several cases, with 8-10 BLFs per phone.
The key is do not get one of those cheap little boxes with a low end Celeron, they mislead on specs, Celerons have no CPU cache, and are like trying to ride a bike through pudding.
Take one of these, put on 4GB RAM, 8GB if you really want to, and a 32 GB SSD, and your done. We use them, they boot up in LITERALLY 30 seconds or less with the 3CX SBC installed, and they support always on, meaning no one has to go press the power button after a power outage, if it has power it will turn itself on, just make sure that is enabled in its BIOS. Auto Power on: enabled, don't set it to last state, it needs to be set to on.
https://protectli.com/product/fw6b/
IT also has a nice massive heatsink on top, and no need for cooling fans that might fail after a few years anyway.
We order these in bulk, 50 at a time, because we use them for pfSense Firewalls as well, we have so far ordered and deployed like 4-5 batches, and only had 4 that were bad(one was dropped), so that's like 1 or less per 100 being faulty.