Try sip gate.co.uk It works for me, from Switzerland, Germany, France and Spain.
I have finally got it working but it took most of the day.
I went with DID Logic. They are so cheap it's not true and the performance is really good.
Even over crappy UK 3G it was "Indian call centre quality" although it may have been possible to improve it with a different codec. Over HSPA or WIFI (ADSL) is works really well.
The VOIP config is a nightmare to get right. There are about 50-100 items to potentially configure. Most of them can be left on default values but if you mess up any of them, it won't work and then all you can do is delete the whole SIP profile and start again.
Having got it running I am now going to stop messing with it. I have just lost the capability to run it over 3G explicitly. It offers only WIFI connections. In reality that is probably what I will use most of the time (VOIP will be used when there is no GSM signal, not so much to save money because I never get through the 100 mins/month) but it is annoying that I can't get the 3G option back. I've been through all the damned menus many times over. It looks like it may offer 3G if there is no WIFI, which I might test later 
Searching the net for VOIP config digs out reams of stuff with people tearing their hair out.
The email support from DID was superb. A very good tech bloke, with excellent written English, and loads of patience
Has anyone here actually used Vonage?
I thought I made it clear that I did. Using an app is no big deal. It is only if you using a deprecated operating system :) that it becomes a problem. They are cheap in my experience but don't have an app for your phone.
Symbian is not today's high fashion I agree but it is completely functional. Amazingly functional in fact; I can browse any Windows PC from it (over a LAN), or run an FTP server on it, and have drag/drop file transfer at ~ 5MB/sec. Great for picture transfer, or moving large files to the SD card. None of this silly Dropbox business, though I have a Dropbox app too. Total access to files (well, as "total" as anything ever needed; for totally total you need to hack the phone) etc...
One issue is that app development stopped at the time the Iphone became trendy, c. 2010, so while most 3rd party apps do function, they are poorly done for the touch screens that came mostly after c. 2009. Nokia continue to update the O/S (they run it alongside WP, for now anyway, till the 808 runs out) but 3rd party apps are mostly dead now. That's why I prefer to use built-in functionality.
I don't know why Vonage keep their login secret. Anybody can get it with a network analyser, and anyway any login in the world is only as secure as the login+pwd somebody has configured. With DID being prepay, the risk is nil anyway; hackers are welcome to the $20 I prepaid
I now have it working, including incoming VOIP calls on a Brighton number which costs $0.70/month
It's possible to configure it so it uses WIFI is available and 3G if not, which is fine.
One remaining issue is this:
Incoming calls stop working after a few minutes.
It has been suggested the SIP Session Interval setting should be changed from 1800 to 0 but this value seems to do nothing. I tried 0, 60, 300, 600, 3600. None of them have any effect.
SIP is supposed to have a keep-alive timer (it's in the RFCs) but it doesn't seem to be functional. This has two issues: the SIP provider may drop the session, and definitely the WIFI router's NAT will close the mapping so the incoming call won't get through.
One could get around the router's NAT auto-closure, for a particular router, by using a 2nd subnet and routing that directly to a WIFI AP i.e. no NAT (and somewhat dodgy security since the AP and everything connected to it will be unprotected) but most WIFI networks won't be set up like that.
There is stuff all over the net (google) and I suspect this is a bug in Nokia's VOIP. The keep-alive feature simply doesn't do anything.
There is another setting called Registration, with options of
When Needed Always ON
which also doesn't do anything whatsoever. If I set Always ON, and come back after a while, I find it has changed back to When Needed. But even if I leave the phone in that menu, and keep it on Always On, the incoming calls still cannot get through after the first few minutes starting from the initial (manual) login to the SIP provider.
The above is Nokia specific but I wonder if anybody here has come across any solution.
The SIP provider tells me even Apple have not solved it for the Iphone e.g. this won't do incoming calls after a few minutes.
It now appears that the Nokia VOIP app is simply non-functional in the keep-alive department
This is fair enough; outgoing VOIP does the job.
It's suprising how many public WIFI networks block VOIP though
Is there a "keep alive" setting that you missed setting?
I am 99.9% sure there isn't any such setting.
What there might be is some "implied" enable. Nokia evidently design their user interface by an Apple-like usability committee and if they can remove 10 useful features to make the 11th really idiot-proof, they do just that.
There is a mysterious setting for the VOIP "registration" which can be "always on" or "when needed" (from memory) and if I set it to "always on" it springs back to "when needed" all by itself, after some minutes or hours. This is either a bug or something else But even when it is definitely in the "always on" setting, the router still shows zero outgoing activity.
I have a feeling that Nokia simply didn't bother implementing this RFC, thinking (as many do) that nobody will want to do incoming VOIP calls on a "phone". On a PBX, sure, but not on a phone on 3G or whatever, sending out some data every minute to retain the session.
But it's also possible that the "always on" setting, to make it stick, requires some other setting elsewhere, e.g. a permanent 3G or WIFI connection. That would be incredibly stupid and the battery would last a very short time. I haven't found such a config option, well not explicitly...
I use the Acrobits softphone you linked to. There is no problem with it receiving calls over a long period, even if the SIP client isn't running and it certainly doesn't need to be doing processing and thus draining the battery. It uses the APN (Apple Push Notification) service to signal to the phone when a call is incoming. Accepting the notification launches the app and answers the call. It uses the one permanent connection that all apps use for push notifications, so whilst that does use battery and data it is relatively little and isn't more and more with each app - all apps receive their notifications using that one connection. Given that most people have that running for push email and other notifications using it for incoming SIP calls is no additional burden.
It works very well. They do offer some other modes such as only receiving calls when the SIP client is the foreground app, but they're pointless really.
DIDLogic is an unusual SIP trunk provider. The prices are really really low, but there are quite a few services they don't offer (like presenting other caller IDs that you own for outbound calls unless you're a big business customer, number porting in, or out, etc). Despite a lot of rhetoric about how great their SIP termination is I've not noticed in being any better or worse than any of the other good quality providers. Their approach to support, FAQs, etc. is amusing. For example, in response to FAQ "My number does not work" they say "All DIDLogic numbers work".
DID present my mobile phone # as the CLI. I guess they would also present the geographical number I bought if I wanted that, but obviously that would be pointless unless incoming calls actually worked.
One thing I like about DID is that they are really cheap and sticking say $20 in there via Paypal will last longer than I care for. I don't want yet another $10/month (or whatever) contract, just to solve a specific narrow issue (being able to make calls when there is WIFI but no GSM).
I've given up on incoming calls - unless I can find some easy solution. I can see how Apple solve the NAT etc keep-alive issue, but in their case it doesn't hammer battery life any more than normal because - just looking at the open NAT sessions on the router here - both the Iphone and the Ipad are online almost continually anyway, 24/7.
But I don't want an Iphone. I can copy/paste between my 808 phone and my PC, over wifi, which is priceless. Apple are still busy pretending that the device does not need, and therefore "does not have", a file system! Accordingly, some of the apps go to mind-bending (well, my mind bending ) lengths to present what is a trivial directory structure, using obscure symbolism. Look at Print Bureau for example.
DID's support is indeed interesting
What is puzzling is that 3G (~100kbits/sec UP) is only just (and usually not quite) about good enough for unbroken VOIP. After all, ISDN runs at just 64k. It must be the extreme network latency that does it. WIFI and HSPA are usually perfect of course. Different codecs don't seem to do anything.
