We had VOIP phone service and an MPLS connection between our main office and a branch office installed recently. The genuis who set things up created an internal network at the branch office using 10.11.0.0/24. Our main office has an internal network of 10.10.0.0/16 The two networks are not communicating well. We have tried to setup a route using the firewalls. Some things work, others don't. Is the subnetting the problem? Do I need to have the provider change the router settings (I can't access their router)?
In general, if you create a subnet, the gateway on that subnet needs to have a route to any other subnet you want it to be able to access. So, the gateway (router) on each subnet needs to have a route to the other subnet. There are a number of ways you can do this - you say you have a route in the firewall somehow, so assuming the firewall has the ability to pass traffic to the other office's subnet, you could use the default gateway to route traffic to the other subnet through that.
Because you say some things work and others don't, it's possible that you have not got a routing issue. but a firewall issue.
Needless to say, I would avoid applying NAT between the two subnets, as they are both in RFC1918 space; the typical solution would be to use the MPLS link between them and have the routers simply route.
It's also important that both subnets have routes to each other; this is another reason why a few things may work but the vast majority will not.
Try running a traceroute between hosts on either subnet to see where the breakdown is.