Can't connect IP phone to server

I've recently purchased a Siemens Gigaset S450IP phone (UK) The phone works fine as a normal house phone, but I have been unable to get it to connect to the server. I followed all instructions on setup and checked settings on router, but to no avail. After days of trying and getting my son, who works with computers, to try and sort the problem, we came to the decision that the base station must be faulty or there was a problem with the firmware. Unfortunately as I can't connect to the server, I can't update firmware. Thinking it might be a problem with the base station I obtained a replacement phone. However the replacement has the same problem.
My router (safecom SWART2-54125) appears to be working and registers the IP address of the phone, but if I do a ping test it is inconsisent and rarely goes through without dropping packets. Even thought the router is obviously assigning the phone with an IP address I cannot access settings through the web browser as the manual says. I am getting messages on the phone saying, "Provider registration failed", "cannot access server" and "check IP settings". We have altered settings on the router to see if it makes any difference, but doesn't. I've changed settings to static IP address, have tried resetting the base station, handset and router. Thinking it could be a connection problem, I changed the ethernet cable. None of these made any difference. I'm at a loss and wondering if it is something to do with the settings, or a problem with either the router or the phone. HELP