Andrew,
As a installer here is my feedback based on my experience and what we do for our customers.
Keep in mind we have not tested every setup. We just know the way we do it is rock solid.
1. Cloud or on Premise?
We tend to do more on-prem vs cloud. The "Customer" in this case has more control over their system if it is on-prem and we seem to have better results over all. This is also a bit dependent on your deployment type. PRI/analog is not as easy to do in the cloud.
2. Which gateways/trunks are best for either solution?
Patton gateways hands down have been the best gateways made in my opinion. For both PRI and Analog use no other.
3. What is the least problematic handset?
We use almost 100% yealink phones. We have customers out there who have used the same models for clost to 10yrs and they are still moving along. Avoid the cheaper model phones, not that they are bad.... but you get what you pay for. We try to stick with the T4 and T5 series phones with Yealink. They cost a bit more but the feel and quality is worth the extra $
4. What pitfalls to avoid with 3CX?
This one is a little hard to answer publicly as i am sure Nick will read it. But I'll be honest with my response. One area we at times struggle with is not necessary the system its self but support. Not sure where you are from "I am in the USA" and there is a bit of culture difference i think. 3CX support is at currently to my knowledge based out of Cyprus "Europe". At times you will get guys on the phone who seem very quick to jump to conclusions that you are wrong and their way is the right way. With VOIP/3CX there are always multiple ways to do things when a customer needs a creative solution. I'm not at all saying the 3CX support guys are bad, they know their stuff and can be helpful. Just at times they are very intimidating when they jump right to you being wrong. Just be prepared for that.
Outside of support the product has been fairly rock solid. We have been selling/installing it for hmm 5 yrs i think with little issues overall.
5. Which version of 3CX?(windows or linux? and also which build version)
Our deployment base is 100% windows. (Always the newest build. Excluding Beta of course)
Reason we do windows is we are familiar with it. All my co-workers including myself have windows server backgrounds. So for us this was a big part of why we picked 3CX as our voice platform. We already know the server side no need to learn new CLI or such for a Linux.
Here is our Magic Mix. The standard configuration we start the conversation with for new customers.
3CX Professional (we never do standard)
SIP Trunking (We use NexVortex exclusively. They used to be supported by 3CX until recently. 3CX will hopefully bring them back)
*PRI (If SIP Trunking is not a option we go in with PRI. We try very hard to avoid analog installs. They are a headache.)
Yealink T46 (This is our "Standard" phone model we start the conversation at. 888voip is a good distributor if you don't have one)
Yealink CP860 (This is the entry level conference phone by Yealink. We have a lot of customers with conference rooms)
Snom PA1 (This is a paging adapter made by Snom. It allows you to connect into existing Paging amps. Good option for the price)
HP/HPE/Aruba. PoE Switches. (the Yealink/Snom equipment is PoE based. We only do installs with PoE. We don't use power blocks)
If you got any specific questions fee free to PM me.