iPhone 3GS-Can use existing US t-mobile SIM and German O2 SIM cards (for phone calls) without buying data plan

I currently use 2 phones to make calls: one t-mobile locked phone for the US and one O2 locked phone for Germany, both without data plan.

Can I upgrade using just one phone (factory unlocked iPhone 3GS)  and continue to use the existing SIM cards for phone calls (just switching the SIM cards when necessary), but use free WIFI spots worldwide for internet access/data transfer?

Thanks Michael