Vbuzzer was the first paid voip provider I tried for a did number.
(did number = Direct Inward Dialing, it is the number used for your phone that is accessible from the public telephone network).
The features offered are like this: a cheap phone number with unlimited dial in calls $2.28 for USA and
Canada numbers, cheap cost per dial out calls ($0.015 per minute for USA and Canada), and a lot more than other company offers like call display and call forwarding, a softphone that shows you your balance and let you put MSN contacts in the buddy list. They don't offer video calls even if you use another softphone. This is one of the bad things about vbuzzer. Another bad thing is that forwarding is not always work as it should, and you can't forward the calls to a sip uri.(si uri it is the full internet address for a sip phone that is accessible anywhere in internet). Other inconvenient is that they don't use the standard 5060 port for the server registration and apparently even the client port is restricted to tcp port 80, 5060, or 5090 which is bad if your sip phone does not support ports less than 1024 (like mine).
My conclusion is that if you want to use the computer to make and receive calls, then vbuzzer could be a cheap choice, but if you want to use a dedicated sip phone then you might want another provider.
Voxalot has something very interesting. First it allows you to have your number for free, but not a public phone number, just a sip phone number, but you can do a lot of things with that, you can forward to another number, you can have the voicemail, and you can connect 2 phones from their website.
Second you can have multiple dial out providers from your voxalot account. This is the best part since some voip providers has smaller fees for some countries so you can create your own algorithm to dial to. For example you are in country 1 and you usually call country 2 and country 3, and you have 3 different providers that have good price (and service) for each country but not one for all. Now you can use your voxalot account to dial any country and the provider will be selected automatically. You can define a dial string i.e. a country prefix, so, if your dialed number will start with that prefix, voxalot will know which provider will chose. This works for free for dial out and also can work for dial in for a small yearly fee. With dial in, you can receive calls from different providers on the same phone (your unic voxalot number).
Third great feature is that you can create speed dial numbers which will also select the best provider that you put in.
Forth feature is that even with no provider you can still receive calls from the public telephone network if the party who's calling dials a sipbroker access code and the voxalot number. You can see that sipbroker has a lot of access numbers in a lot of countries and cities, and it has almost all voip providers that can interconnect with.
Unlimitel is another voip provider that I tried. It has about the same price as vbuzzer per did number and dialed calls, but they charge the same price per minute per dial out and dial in calls (for pay as you go plan). In addition, they offer 911 service and the service is much reliable than vbuzzer and it uses the standart tcp ports 5060 for server registrations.
The only problem seems to be the incoming calls that are charged, but since you have a phone that is a sip phone the you can configure it to receive calls for free if they are coming from internet. So, for that you need a system that will connect the public phone system with the internet. I just talked about sipbroker, it will do the job perfectly. If anyone want to call you for free, they will have to do the following: dial the local sipbroker number, dial the sip network prefix and then dial your sip number. This procedure should work for every voip provider registered in sipbroker database.
If you don't have any voip provider registered you can still receive calls if you have your ip phone (ata or softphone). You just need to configure the ip phone to receive direct ip calls, and then you should register your ip address to a dns service like dyndns. Once you have a dns name for your ip address then you need to forward port 5060 form your internet gateway to your ip phone and you are done. Now anybody can call you using your sip uri (your defined number on the phone plus your dns name for your ip address). If you want to receive calls from the public phone network, you can register your sip uri to sipbroker and you will associate it with a phone number alias. Once the people will reach sipbroker access number they can call your alias number and your phone will ring-all for free unlimited.
Article source: millionguides.com