Royal2000h,
I did a quick check of about 12-15 country calling rates for Roamless vs. Google Voice. It looks like Roamless is pretty close to GV rates, or worst cases I saw, Roamless may be 10-40% more expensive. In most cases, the difference is only say, $.02-$.20 per minute, depending upon the carrier called. I have T-Mobile USA service and so I won't use anything over the$.25 a minute rate I get from TM.
I was on a US Mobile $10 a month plan mostly because it had free calling internationally while roaming, but just cancelled it when I subscribed to Roamless. Both my Roamless US data service on AT&T at about $1.50 Gb, and the low calling rates seems to be a better choice.