I’m suffering the same problem after switching my carrier to Mint Mobile (USA) which uses the T-Mobile network. The phone is a OnePlus “dre” running /e/os 2.9 with a physical SIM.
Calls work both directions but SMS only works outbound. I’m going to call my carrier today and see if they can help me. Hopefully they won’t ask me to mess with the SMSC number because I don’t see it anywhere on my /e/os phone.
Fortunately my /e/os phone is a secondary so I’m not under any pressure to resolve the issue. My primary phone is a Motorola Razr+ running stock Android with T-Mobile and I can see the SMSC number under Messages->Settings-> Advanced, but I don’t see anywhere to modify it.