I run a 2860 and a two times 2860n Plus on three different sites. I haven't tried any with VOIP built in. Two of the sites have Snom VOIP phones connected via the Drayteks and there are no issues whatsoever. It's because we already had VOIP phones that I didn't get the 2860V range.
One 2860 and one 2860n Plus have secondary WANs configured to come up when the VDSL2 connection is down through a 3G USB modem. This works well. Neither uses an Openreach modem. The VDSL2 connections are very stable indeed with good throughput.
The 2860n Plus model had problems with wireless when using multiple VLANs using the original 22.214.171.124 firmware. Connections would become randomly unresponsive for periods of over a minute (this also affects 2850n routers).
I am currently running a Beta firmware sent to me by Draytek which fixed this (it was an ARP issue) but it made 5G connections much more difficult to achieve when using a Macbook Pro (I haven't tried other clients).
There are two new firmwares out in the last month but neither mentions this as either a problem or an improvement and I haven't had time to test the new firmwares yet.
A problem I have experienced on the 2860 with 126.96.36.199 firmware (but haven't tested on the 2860n Plus) is that when using multiple VLANs, DHCP only works on LAN1. LAN1 connected clients can't be blocked from connecting to the router's admin web interface (all others can). I have yet to raise this with Draytek but will do.
I needed to run DHCP for Barclaycard supplied Ingenico card payment terminals so ended up using a Raspberry Pi as a dedicated DHCP server on the card data VLAN to get over this problem. Now anyone who manages to plug into an ethernet port won't get an IP address unless they know the MAC address of a terminal and clone it - and even then they won't get admin access to the router.
I also run a 2850 with 3G/USB WAN failover. This is very stable except that after about three months or so of continuous uptime it locks up and all interfaces become unresponsive. To prevent this happening I have scheduled a weekly reboot and have subsequently applied this to the 2860n routers even though I have yet to experience a similar lockup.
Generally I like Draytek routers once the bugs have been sorted out. If they did everything they were supposed to out of the box, they'd be almost brilliant. I say almost
because my one major dislike is that the configuration can't be edited offline as with most other routers, business grade or consumer. This makes building complex configurations very tedious indeed and I only run complex configurations
It's going to be a couple of weeks at least before I have enough time to test the new firmware on the 2860n Plus as it takes about six hours to do a thorough test (and I've done about six in the past). When I have, I shall give you an update.