Hi Paul,
I have 3CX installed in SBS03, 08, and 11 networks, and most also use TS for remote desktop services. While some of these are hyper-v children - the 3CX system is in all cases installed on dedicated hardware.
I've been working with SBS since the 4.5 days, and we know that it's optimized as a suite, and we know that additional roles beyond what is designed to do can complicate things. It can be done, but I'd not, and don't. I wouldn't use the RDS (TS) OS to host the phone system either - that defeats every kind of best practice for RDS which has a single and extremely important role.
In smaller networks (up to 8 SC), Windows 7, larger networks Server 2008 r2 (any flavor). The Win7 3CX run a mix of Abyss and IIS, while the WS2008 all run IIS. Win7 installs run on relatively inexpensive laptops (they have build in batter backup, sound card, and have a small footprint). WS2008 installs run on HP Microservers, typically 4 Gb of ram. These little boxes are almost silent, have a small footprint, have onboard RAID1, and can provide some file storage as well if needed.
While SBS customers are used to getting a lot for a little, and everthing and everyone having more than one role or purpose ;-), I think it's wise to keep the phone system independent of other servers.
Your 15 users and 8 lines (possibly a lot for 15 users, BTW) will run well on a laptop with Win7. If you have SBS Premium and still have the extra Server license available, then WS 2008 R2 is also an option.
My 2c, hope it helps you decide
.
