You didn't say where you are calling, so I have only general advice:
Usually, a Betamax company will be least expensive. Quality and reliability are not the best, support is almost non-existant, and some only accept limited forms of payment. However, lots of folks like them, because they can talk and talk without worrying too much about the meter running. A comparison list is here:
http://backsla.sh/betamax . Chart prices are in Euro cents per minute. Some also add a setup fee and/or charge more for calling card (as opposed to VoIP) access. Some offer "free" days to low cost destinations, but the rules are complex. Check individual Web sites for details.
Next step up, look at Localphone, CallWithUs, Future-Nine,
VoIP.ms. All have decent support. Localphone is similar to Rebtel in features and provides a small credit at signup, so you can test without making a payment. The other three provide a choice of routes (on Web site or per call); if you have trouble or poor quality, you can retry with a more expensive carrier.
Voxbeam (a Localphone brand) is quite inexpensive and their Premium route is generally high quality. Unfortunately, they authenticate by IP address only, so there is some hassle using them. Small test credit provided at signup.
For excellent quality, reliability and support, look at Callcentric or Anveo.
All of the above providers accept unregistered calls, so you can use them in VGx or speed dials in your OBi, and you can configure multiple OBi devices to use the same account (though some limit you to two concurrent calls on a given account).