This will work over the WAN but crucially your receiving firewall will need to cooperate and you will need to mod the .spd file according to the setup at the transmitting end regarding NAT. Assuming the target PC is NATed behind a firewall, the receiving FW (the target IP address) will need to admit and pass on traffic (udp or tcp depending on what the script does) to the target port.
If the sending phone is behind a firewall, then that firewall will provide the IP for the .sdp file. Thus the auto-generated. sdp will not be suitable. Trying this over mobile internet will likely result in a NAT-like situation, where the APN assigned IP of the n900 will not appear as the transmitting one.
So some fiddling around will get it working. And tcp might prove more robust than udp depending on the firewall setup.