Best practice amalgamating two networks

I currently have two separate networks in the same building and a remote RED connecting to network A

Network A, uses a fibre BB connection, 10/100/1000 switch, 20x PCs/ servers/ printers, on 192.168.100.1/24 through a Sophos 110/120 UTM
Current setup on Network A  =  Modem>UTM>switch

Network B, uses a fibre BB connection, 10/100 POe switch, Asterisk/ freepbx server, a SIP trunk and 10x cisco handsets on 192.168.1.1/24 (all static)
Current setup on Network B  =  Modem>router>switch  (router forwards ports 5060 UDP and 6000:40000 UDP ONLY from SIP providers IP to voip server)

I need networks A+B to share the new superfast broadband.

My original idea was to use the DMZ in the UTM then a separate router to handle the voip network, but I don't think this is the best way. I could also install a router before the UTM and not use the UTM as the PPOe negotiator. But it would be a shame not to use the UTM fully. 

How can I get the UTM to manage both networks? I can re-address the voip network to match the data one but I'd prefer to separate the traffic ( i'd rather not have the voip packets travelling round the data switch)

The reason there are two fibre lines is that the original setup required the voip server and data networks to have separate subnets and the Asterisk voip server was really picky about the NAT when it was re-routed from one router to another to change the subnets (this was prior to having the UTM)

There is no current requirement for the Remote RED site and it is down for the time being but I might reinstate it later, possibly with voip if possible.