For those who might be interested:

During the last days we built a B2BUA based on Freeswitch and Raspberry PI Ver B which connects to a central server via OpenVPN in order to encrypt voice traffic and circumvent NAT issues. We plan to use it during travelling and to have VPN access in the home office. The costs for this are relatively low (~75$).

We installed

• Raspbian
• Freeswitch compiled native from GIT (this really takes a while)
• OpenVPN
• Dnsmasq for eth1 side
• isc-dhcp-server for eth1 side
• tftpd-hpa for eth1 side
• a second LAN interface eth1 was done by a Delock USB 2.0 to LAN interface (about 20$), works out of the box
  
• iptables firewall for NAT traversal
  

This worked pretty well. We were not sure beforehand, whether the USB to LAN interface would work nicely, but in fact it did.

CPU usage during a single call is
top - 10:47:34 up 39 min,  1 user,  load average: 0,64, 0,40, 0,24
Tasks:  70 total,   1 running,  69 sleeping,   0 stopped,   0 zombie
%Cpu(s):  6,5 us, 12,1 sy,  0,0 ni, 64,9 id,  0,0 wa,  0,0 hi, 16,5 si,  0,0 st
KiB Mem:    448776 total,    94656 used,   354120 free,    13096 buffers
KiB Swap:   102396 total,        0 used,   102396 free,    42464 cached

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
 2785 root      -2 -10 28328  14m 4844 S   5,3  3,2   0:30.45 freeswitch
 2651 root      20   0  5800 3052 1848 S   3,0  0,7   0:07.73 openvpn
 3051 pi        20   0  5096 1256  968 R   0,7  0,3   0:01.97 top

4 GB CF card was about 75%. Codec was G711.
The load was mainly generated by the USB-to-LAN adapter. Maybe someone has a better solution for this?

At the end we were impressed, how easy this was to set up.
So now we have an easy VPN-Router/B2BA which allows us to

• connect a PC somewhere and tunnel all traffic via VPN
• connect a SIP phone somewhere and tunnel all VoIP traffic via VPN

We did this with an OpenWRT Box before, but this is much more easy to handle.

Best regards
Peter