Zoiper/VoIP calling programs

I am using zoiper to connect to voip provider in Long Island/NY so I am outbound from US via wifi  and front end with zoiper.  I use it here in VN, France,  UK, Thailand , Hong Kong where ever I travel, the call is clear and it's cheaper than roaming.  My Voip doesn't provide SMS therefore I use Google Voice for SMS  in US and voice forwarding to my VOIP/voice mail to my client on my smartphone here in VN. Hope that's help

Thanks for the reply. Could you tell me how you set it up or who helped set it up for you? That would be most useful. Thanks.

My voip in long island is callcentric.com.  they give free inbound number for free as well voice mail. Their online support gave a very detail how to set up your client like zoiper or csip.  You also need to put some money in ( I put 10$ and it's last forever no expiration date only when you use for outbound call ).
I setup another number with google voice. But google voice is not a voip provider. It's just a call management ( no need to put money in google)
Then you forward all cal to callcentric.
here how it's will work out:

I can sms any number in us canada for free with google voice. The receiving will see my google number on their sms. They can sms me via my google voice number and I will receive a email tell me the msg as well I can read in my google voice aps on my smartphone. All that is free.

now from the voice side.
1. people call my google number and it's will be forward to callcentric which in turn transfer to my zoiper client via internet.

2. People call my inbound callcentric number and it's forward to my zoiper client same as above.

For both case if I don't pickup it will go to my callcentric voice mail and I get email notification from callcentric.  I just use zoiper and dial *123 and get to my callcentric voice mail.  All that's free also.

in callcentric you have a option to limit outbound call for which country you can call. Mine is Europe and south asia, us and canada is open by default.

when I travel I don't roam just find a wifi spot ( cafe hotel some airport where I transit ) and call people like regular phone for pennies.

People will see my callcentric number on their call ID.

That's my setup.

The configuration of zoiper for callcentric is straight forward from their support

You can also use fixed client like grandstream 286 etc... device where it connect to your LAN line ( as regular desk phone ) similar to magic jack setup.
hope to answer you.

I designed and setup by myself. My background is IT telecom in US.
I designed and run data center, switch, IVR etc...