Our calls keep dropping. Is this likely to be a hardware issue or a network issue?
Most likely either a network or connectivity issue. Phones normally work, or they don't.
Making a call and then it dropping after a certain amount of time normally points at the firewall on the router being the issue, the firewall is closing the port while you are on a call.
I see you have logged a ticket with Support, they are going to ask you for some exact examples so they can do a SIP Trace and see where the issue is originating.
Please make sure that SIP ALG is disabled on your router as a start.
Please deal with this issue via the ticket from now.