[CrackBerry Question]

Purchased an unlocked BB 9550 used, do I need a Verizon sim card? I am currently a Verizon customer

[Rustybronco]

It looks like the answer is no, you do not need to use a Verizon sim.
BlackBerry Storm2 9550 - Full phone specifications

AT&T...

Frequency range Band number Protocol Class Status Notes
850 MHz GSM/GPRS/EDGE 2G Decommissioning by 2017[30]
1900 MHz PCS GSM/GPRS/EDGE 2G Decommissioning by 2017[30]
850 MHz CLR 5 UMTS/HSPA+ 21Mbit/s 3G Active[31]
1900 MHz PCS 2 UMTS/HSPA+ 21Mbit/s 3G Active[31]
700 MHz Block B 17 LTE 4G Active[32] Main LTE band providing complete coverage
1700 MHz AWS 4 LTE 4G Active[33][34] Additional LTE band for more bandwidth in select markets
1900 MHz PCS 2 LTE 4G Active[35] Additional LTE band for more bandwidth in select markets
2100 MHz Block B 4 LTE 4G Active[36] LTE band providing down-link access
2300 MHz WCS 30 LTE 4G Active[37] Additional LTE band for more bandwidth in select markets

T-mobile...

GSM Network

Frequency Band Band number Radio Interface Generation Status Notes
850 MHz 5 GSM/GPRS/EDGE 2G Roaming Not operated by T-Mobile, except for CMA629 in SC after the acquisition of SunCom. Competitor 850 MHz networks are accessible via roaming agreements.
850 MHz CLR 5 UMTS/HSPA+ 3G/4G Roaming Not operated by T-Mobile. After the failed acquisition of T-Mobile USA by AT&T, part of the failed acquisition was a 7-year AT&T 3G/4G roaming agreement.
1900 MHz 2 GSM/GPRS/EDGE 2G In Service Most of this spectrum will/have undergone refarming for HSPA+ and/or LTE services.[67] (More Details See 4G LTE Upgrade)
1700/2100 MHz AWS 4 UMTS/HSPA+ Up to 42Mbit/s 3G/4G[76] In Service Has marketed 4G HSPA+ as "4G" since 2011. Covers 230 million+ POPs as of March, 2014.[67]
1900 MHz PCS 2 UMTS/HSPA+ Up to 42Mbit/s 3G/4G In Service/Building out Refarming 4G HSPA+ traffic to this band.[77] Covers 203 million POPs as of Oct, 2013[78]
1700/2100 MHz AWS 4 LTE 4G In Service/Building out Together, both bands cover 260 million POPs as of December, 2014 [79]

PCS LTE is typically deployed in rural areas to reuse GSM antennas. PCS LTE is also being rolled out in urban areas to increase LTE capacity.[80][81][82]
1900 MHz PCS 2 LTE 4G In Service/Building out
700 MHz Block A 12 LTE 4G In Service/Building out [68] Spectrum acquired from Verizon Wireless and other smaller license holders[83][84]
5 GHz No assigned band LTE 4G Planned Unlicensed LTE planned to be launched in 2015[79]

From wikipedia.