I'm glad you mentioned faxing.
If you are looking at VOIP, faxing is a mixed bag. If you are getting your lines via SIP, you usually need T38 support on the provider side and on your ATA.
I have it working with my PRI-->Internal SIP (3cx)-->ATA hooked to the fax machine, but there are some fax numbers that we have problems with even after having slowed down how fast the fax machine talks.
I have considered going back to a fully analog line or doing something like FaxxBochs: »
www.rockbochs.com/compon ··· axxbochsHowever, our setup works well enough that so far we have not changed. But this is a potential area of pain for you that you should be careful about.