For IFR flight plans I use a combination of
EuroFPL and the UK NATS AFPEx facility.
This morning I filed a flight plan using AFPEx. The reason was that there was a reasonable expectation of the departure being delayed but I would not have internet access at the relevant time, and AFPEx have the telephone helpdesk.
Sure enough it needed delaying so I phoned the helpdesk, which on past occassions was always totally helpful with these kinds of things.
This time I got a very "short" individual who told me in no uncertain terms that delaying flight plans is the job of the airport and that if I had a phone to call them with, I could equally call the airport..
He then promptly disconnected the call so I did not have enough time to explain that this may not be possible for various reasons, e.g. no English speaking personnel available, no published (or working) phone numbers, no 3G and the seemingly random 3MB Java app download (which incidentally I got hit with last night; luckily still in the hotel, on ADSL, and it still took about 20 mins to download) takes over half an hour on GPRS, etc.
To ensure this was not just one individual who fell off the wrong side of his bed, I phoned again 10 mins later and got someone else who was a lot better but confirmed it had been the Manager I spoke to earlier, and yes the helpdesk function is limited to internet issues and maybe helping somebody in the middle of a farm strip.
Having always spoken highly of AFPEx, this was news to me and it highlights the need for getting one's flight plan arrangements to be well sorted, with a laptop with mobile internet on it to be always available, and have at least one other flight plan filing/managing service apart from AFPEx but one which you may have to pay for. I believe EuroFPL will soon have SMS facilities for doing this stuff, which is really the most general-purpose way. I can't recommend Homebriefing.com anymore since they bloated-up their website and their helpdesk has been a bit "random" too.
It will probably suit NATS down to the ground to have people paying for 3rd party solutions, while NATS have nominally discharged their ICAO obligations with AFPEx, having done the admittedly businesswise-obvious thing of shutting the FBUs.
In the end I got the mandatory-handling handling agent to delay the flight plan, which it subsequently turned out he failed to do, presumably because he could not speak English and just said Yes Yes Yes. Fortunately ATC had not binned the flight plan (as would normally happen after an hour or more's delay) and were able to re-file it.