Acording to GSMarena.com, the A510F has only one LTE band used by T-Mobile. Band 5 (850 MHz), which is only used in Myrtle Beach, SC. It has only one UMTS (3G) band used by T-Mobile, band 2, which is not used for UMTS everywhere. It has 2G compatibility with GSM-1900. In some markets (e.g. NYC) this phone would give very poor performance.
The Sony appears to have essentially identical bands and would perform equally badly.
Each appear to be phones designed for the European market with little concern for the bands used in North America.
Thanks a lot for your kind replies.
Actually we live in 11801 and since we have modems for internet at home / work, we use the devices mostly as phones and less for internet.
We hope the provided information helps.
If you get a lot of dropped and missed calls in that market, it's because the phones are stuck on a very thin GSM layer. In the entire NYC market, it really helps to have a phone that's fully compatible with all T-Mobile's bands and standards.