There are 7 versions
1) 640 XL single-SIM (no LTE)
- source: Microsoft Lumia 640 XL - Smartphones - Microsoft - Global
- model number: RM-1066 (source: nokiapoweruser.com)
2) 640 XL dual-SIM (no LTE)
- source: Microsoft Lumia 640 XL Dual SIM - Smartphones - Microsoft - Global
- model number: RM-1067 (nokiapoweruser.com)
3) 640 XL single-SIM LTE
- source: Microsoft Lumia 640 XL LTE - Smartphones - Microsoft - Global
- RM-1062 does not support USA LTE bands (unverified)
- RM-1064 does support USA LTE bands (verified)
- model numbers: RM-1062 , RM-1063 (USA ATT), RM-1064 (nokiapoweruser.com)
4) 640 XL dual-SIM LTE
- source: Microsoft Lumia 640 XL LTE Dual SIM - Smartphones - Microsoft - Global
- model numbers: RM-1065 , RM-1096 (both versions verified to work with USA LTE bands)
- for ATT USA, visual voicemail does not seem to work for either RM-1065 or RM-1096
See
http://forums.windowscentral.com/lumia-640-xl/361557-lumia-640-xl-versions.html