Faster than even our 4G LTE
The answer to your question is: nothing. There are no agreements, partnerships or any other connections between Verizon and T-Mobile. The main reason there is not is that our networks are completely incompatible. T-Mobile uses GSM (sim card technology) for their network and we use CDMA. Yes, newer 4g LTE Verizon phones use sim cards but only for international roaming. Hope this helps.
Very tech savvy and very thorough. If I need to further explain something or you have more questions to ask...
No the Sim accesses 4g or international capability...