I am using voipcheap.com from the betamax group. A bit of warning. Don't expect any support from betamax group.

Most likely you will not need support though.

I pay EUR 10.00 and have 400 minutes of free calling to almost all landlines in the world per week free of charge for 90 days. If I call mobiles it will deduct money from the 10 EUR. If you do not call much you can use the 10 EUR after the 3 months are over. I call ALOT and will typically use 10 EUR per month (as I also call mobiles) this is why i have about 300 "free days" which means i can call land lines free of charge for 300 days...

Call quality is suberb. I use a N95-8GB (for travel) with built in sip client and a Gigaset C475IP (for home)