This is very much looked at from the American perspective. I live in the Netherlands, and things are somewhat different here. Firstly, I don't know of any credit cards here that give you cash back. On the contrary, it's not unusual to pay for your credit card (in my case, it's part of the service package my bank sells me). Also, paying by debit card is far more popular here. And some webshops will charge you for that -- and some of those accepting bitcoin will not charge you or less, resulting in a clear discount.
Examples:
If I buy bitcoin, I do so at Kraken for a 0.35% fee. Sending fiat via SEPA to Kraken is free, and getting my BTC out of there is 0.5 mBTC per transfer.
However, I have to stress that most webshops don't explicitly make the debit card charges visible to the customer. Once they do, it will make a difference. After all, €1 on your €20 pizza is 5%. And €0.45 on your €10 voip phone credit is 4.5%. Way more than the 0.35% for getting bitcoins.
Can you provide your Trunk Settings from the web interface under Connectivity > Trunks > Edit Trunk > SIP Settings > Outgoing/Incoming. Redact your username/password/DID.
This looks like it has the settings for trunk setup. This post indicates you'll need to use sip1.voipbuster.com as sip.voipbuster.com may be depreciated. Not sure if you're forced to use a STUN server. Try to avoid it if possible.
You'll also need to have a default Outbound route set up under Connectivity > Outbound routes. I use the following dial patterns:
1NXXNXXXXXX 911 NXXNXXXXXX
Route Settings should have VOIPBuster selected for "Trunk Sequence for Matched Routes".
Register an account at Betamax VOIP services (such as www.voipbuster.com ) and deposit a few bucks of prepaid credit. When you reach your destination, get a prepaid SIM card, then follow instructions here.
You'll end up calling a local phone number at great rates, and the automated operator will ask your for a number you wish to call - you'll be patched through and be able to call all around the world.