|Well, I work as a network technician and we employ VoIP at work.|
I'm not sure how it works in a small environment as we have our own PBX (phone) switch and use VoIP controllers that interface with our PBX switch. But if I understand it correctly and you're getting your VoIP through your ISP or a 3'd party, the VoIP traffic should flow from your network to the VoIP providers VoIP controller and from there to the telephone system.
I can tell you this. Our VoIP traffic never goes near our DC's. It doesn't have to. It goes from VoIP phone to the VoIP controller to the PBX switch and from there to the telephone system.
Your's shouldn't either unless your DC is your gateway to the external network (internet).
What you could do is get the TCP/IP configuration from your VoIP phones and from your two DC's and paste it in here and I could tell you if the traffic is indeed flowing through your DC's.
I do have to ask though, what exactly do you mean by "hitting" and explain also how you know your VoIP phones are hitting the DC's in question? I confess I have a certain amount of curiosity as to how a guy who's "not tech trained" figures out that his VoIP phones are "hitting" the DC's.
Oh, as to the "Some of my PC's in A are also hitting DC B but it is just every once in awhile." that's not unusual or unexpected. If there are services or files on DC B that PC's in A need to access, if follows logically that they will 'hit' it.
Using VLAN's to segment your domain is all nice and dandy (we VLAN extensively where I work) if you have need to. But if you don't, then you're looking at having to purchase VLAN capable equipment (switches/routers etc) and those do get pricey. Just by way of example, our layer 3 high density switches run around $5,000.00 each.