Is a sim box the same as a proxy farm? With the simbox you can rotate the IP of the SIM as you do with a dongle and a SIM connected and then with a program to configure the rotation of that IP? Or is that not possible with the simbox?
SIM BOX
I have not used or tested a SIM box, but I took a quick peek and it appears to be more of a physical SIM hub/switch/router than anything else. A SIM box may have additional features, such as the ability to create a wifi hotspot, but that would still route through the existing hardware (SIMs) and is hardly worth the excitement.
A SIM box will rotate through the physical SIM cards you have installed in it. So, if your SIM box holds 4 physical SIMs, and all 4 of those have nearby cellular networks available, then you can use any of the SIMs you want, or rotate through them. Usually though, this would be used by someone who is traveling, perhaps a student, or has multiple cellular provider accounts. For example, if you have a Vodaphone account as your primary, and a Verizon account, and a SingTel account, and your phone only holds one physical SIM, then you could use a SIM box as a SIM switch/router for the other two (Verizon & SingTel). Then, when you travel to PDD you can use your SIM box to connect to SingTel.
You used the example of a SIM connected to a mobile device through a dongle. A SIM box is just a dongle that allows for multiple SIMs to be plugged into it, and selected.
A SIM box is essentially a glorified physical SIM holder/case, as you could also just unplug each SIM from the SIM box and plug it into your phone directly.
PROXY FARM
This can mean several things. For the sake of our conversation, you are most likely referring to a Mobile Proxy "Farm". A Mobile Proxy Farm is a Proxy that uses mobile devices as exit nodes. The primary reason for this is it is generally more challenging for services to ban end-user cellular IP addresses than traditional wired IP addresses, due to the cellular IP addresses tending to be shared. So if someone blocks a cellular IP being used by their customers, they may also block an entire segment of other "innocent" users at the same time.
If you are looking to rotate through VPN IP addresses there are a number of legitimate providers. If that is not your game, and you are willing to press the boundaries, one solid option would be to repurpose a botnet as a proxy botnet. The quality of this proxy would depend on the type of devices which compose the botnet, or in the type and quality of the botnet, but if the botnet's spread correlates well with your intentions for the proxy, then a proxy botnet may be your best solution.