Hay,
I don't know the software, but the part, which you warn about vps, is freeswitch - that is a telephony module, comparable to the asterisk pbx.
With the asterisk, I can make 1000 parallel calls (maybe more) on my RS8000 server and I don't think there will be a problem with a RS2000 with less calls. Please be aware that the RSxxxx-Servers are also virtualized, but with dedicated ressources (RAM, CPU). The mentioned "bare metal server" is a really dedicated server under https://www.netcup.de/professional/.
(It's funny, that freeswitch don't recommend virtual servers, but provides a full installed virtual machine for virtualbox. And recommends the AWS-platform too, which is also virtualized.)
In other parts, I would follow the recommendations, it seems to be a very special software.
CU, Peter