iPhone 5 is unlocked and will work with Verizon with no sim, or with any GSM network with a sim card. BUT, it is limited to 3G speeds. LTE is disabled.
The SIM card slot may be unlocked. However, inserting a SIM card may mess up settings for the iPhone to work with Straight Talk again. You're better off buying unlocked from Apple and getting a Straight Talk SIM card cut to nano-SIM size.
Follow these instructions to get data and MMS working.:
Yes, if it si A1429
From Russin Apple Website on the iPhone:
•Модель GSM: GSM/EDGE
•Модель CDMA: CDMA EV-DO Rev. A и Rev. B
•Wi-Fi (802.11a/b/g/n; 802.11n,
2,4 ГГц и 5 ГГц)
•GPS и GLONASS
Model A1429 – CDMA/GSM Global Model
This iPhone is the one that CDMA carriers and all European and Asian carriers will sell. Like Model A1428, it supports quad-band GSM and quad-band UMTS with single cell and dual cell HSPA+ modes. This model trades in support for LTE band classes 2, 4, 5, and 17 for LTE band classes 1 (IMT, 2.1GHz), 3 (DCS, 1.8GHz), 5 (Cellular 850, 850MHz), 13 (Upper 700MHz C block), and 25 (US Extended PCS, 1.9GHz). Additionally, it supports CDMA2000 on band classes 0 (Cellular 850, 850MHz), 1 (PCS, 1.9GHz), 10 (ESMR 800, 800MHz), and 6 (IMT, 2.1GHz).