I was using http://VoIP.ms worked fairly well as a sip service, and I believe it works the way you're hoping for. I'm unfamiliar with the provider you mentioned.
I struggled with getting a reliable sip service on my cellphone because of unreliable network conditions. What did help was connecting to a TCP VPN.
In the enterprise I found zoiper worked fairly well.
Linphone worked fine as well for my hobby projects but my roommate at the time didn't like the interface.
Some phones have their own sip client deep in the settings too, that is the ideal method.
Edit: a potential solution if you're looking to just use it for international calls. You could configure the sip service to take calls from your number exclusively and call the number you're wishing to dial. And have inbound calls to that number connect to you.