There's a wide variety of SIP providers. Some are aimed at businesses trying to squeeze more phone lines in, some are aimed at immigrants trying to call home cheap, etc. Take a look around and try different ones (they're cheap, after all) until you find one you like. I will share my experience with 2 of the major providers that I've tried.
Most providers tend to be pretty careful about their customers. For instance, OnSip and Mondotalk require that you call them to confirm a purchase of minutes. I suppose this is to make sure that your set-up is working. This is a good thing because your client may not work with the provider.
The problems that you will see with clients are various. Depending on the provider, I can get certain combinations to register, but then it won't make a call. Or there won't be sound. In one direction. Or the other. Or both.
This may be because the NAT support isn't working, or that there isn't a compatible codec (even though they all support uLaw/ALaw.) Keep in mind, I'm not doing anything exotic. For many providers, I can't even call their own main number, much less a subscriber.
One provider that I've had some success with is CallCentric. They have cheap, simple plans and you can still get a DID number from them. In other words, there's nothing special about them. They're just who I was able to get a SIP stack to work with.
One issue with them is that they feel they need to collect a monthly fee to cover their 911 expenses. This will use up your balance steadily (but not quickly.) I don't make many calls and $20 would last me a long time, except for this fee.
The provider that I'm currently using is Mondotalk. They provide all the usual features. In call testing, I hear a slight delay but it's comparable to Google Voice, or Skype. I haven't tried any exotic codecs. My default works fine (and sounds better than my cell phone.)
Despite having a good client and provider, I wouldn't rely solely on SIP on my mobile phone. The networks you will run into are too varied and SIP's NAT hacks aren't reliable. Running SIP over your cell data may work if you a large or infinite bandwidth/data limit.
You may be asking, "But I see all these providers on the net ? Who's buying ?" Small businesses are using SIP to lower their phone bills. It works for them because they don't have to deal with NAT, and they have a static configuration with a provider that they are familiar with.
Next post: What *does* work reliably ?
