This is what the OP asked:
"can an iphone 5 bought from verizon work with voice and data simultaneously on a 3G GSM network internationally ?"
Shouldn't the answer be yes, provided the following specs are present on the A1429 version of the iPhone 5, and it's the unlocked model as currently touted in the marketplace:
CDMA model A1429*: CDMA EV-DO Rev. A and Rev. B (800, 1900, 2100 MHz); UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz); LTE (Bands 1, 3, 5, 13, 25)
If the user hopped on O2's UMTS network (for example) in Europe, wouldn't they also be using voice on the same network, not only data? And I presume, if you were a verizon customer, that handset would have to be provisioned correctly? Otherwise, since it *is* technically sold unlocked by the big V, would it still work to roam per se in this fashion on a UMTS network (using voice & data)?