Cisco VPN :: 851 / 861 Full Mash VPN Network

We have 4 remote locations, all these locations are connected to Head Office via Site to site VPN tunnels, i have one tunnel to each location form head office,

HQ 10.10.1.0/24  (Cisco 851)
Site 1 10.10.2.0/24  (Cisco 861)
Site 2 10.10.4.0/24 (Cisco 861)
Site 3  192.168.10.0/24 (Linksys RV4102)
Site 4 192.168.1.0/24  (Some Linksys )
 
We are using these VPN tunnels for IP phones at remote locations, Every thing works fine except when some body want to make a call form any remote locations to another remote location e..g site 1 call site 2 there is no audio, to over come this problem we need to a fully mesh network, each location should be able to access each other or in other words we can say that i should be able to ping LAN IP of Site 1 form Site 2, so to get it working i need to create vpn tunnel form each location to each location, in that way we have to create 4 tunnels in each router, or we can have DMVPN, but due to mix of equipments we cant have DMVPN, because only cisco 851 supports DMVPN, not even cisco 861)
 
So my question to all experts is that there is any way i can do some thing in HQ, so that it can route traffic from one remote site to another site, or it can work as hub so that i dont have to create 4 vpn tunnels in each router, because of the cisco 851 and cisco 861, i can have only 5 VPN tunnels, if i will create 4 VPN tunnels in each router i will be ran out very soon,