Any PBX that can communicates with other VoIP devices using standard SIP protocol is fine.
Pretty much all VoIP PBXs out there can do that - from Asterisk through to Cisco, Avaya etc.
The SIP trunk from voip.ms (or any other SIP trunk provider) can be routed directly to the VoiceGuide system - with VoiceGuide receiving calls from it directly, and making calls out over the SIP trunk connection directly.
Your router just needs to be set to send traffic originating from the SIP trunk provider to the VoiceGuide system's IP address.
At same time, VoiceGuide can also connect to a VoIP PBX, and transfer any calls to PBX extensions as required.
If you do want to route all calls between the SIP trunk provider and VoiceGuide through a PBX first then this is fine, but each device in the path adds a little bit of delay to sound traffic and another point of possible failure (and if any sound trans-coding is done by the device then quality of sound would be degraded), so if the SIP trunk is going to be used for calls handled by VoiceGuide the better solution is to have VoiceGuide system communicate directly with the SIP trunk provider, instead of having all the calls go through a PBX as well.