CCRT700 are, as said, GSM 900mhz only. The CCRT2008 were Dual Band GSM 900 and PCN 1800mhz ( later to be known as International 2G GSM 1800), so the Orange and T Mobile Sims would also work on the CCRT2008.
Probably also worth mentioning that a lot of the Bandwidth on these Bands have been Re Farmed for 3 and 4G use now, so GSM Coverage now is at its bare minimum requirement and obligations under International Roaming Agreements.
It's also planned to allow the GSM Service to be concluded and switched off by 2025, so potentially limited Life left with these HU's now, although the UK does use this Service for a lot of Meter Readings, Remote Status Messaging for Utilities and Transport, so what the UK may do is still being debated although you don't need Digital Voice Bandwidth just to send a String or a few Packets of Telephone Numbers.
I believe the OP issue is probably using a PAYG SIM that has fallen off the System due to the 6 Month rule of not making s Chargable Call or Text within that period.