[OnePlus 5T  ROMs, Kernels, Recoveries, & Other Dev] flotwig:
Since the issue with sending SMS is still persisting, I thought I'd share some logs from logcat. I believe these are the relevant modem logs from when my SMS sending fails.

My failure is happening on Mint Mobile, a MVNO of T-Mobile, with VoLTE enabled (I can't figure out how to disable it). This happens with SIM1 or SIM2.

04-26 18:28:09.657 D/SmsController( 4193): sendTextForSubscriber iccSmsIntMgr Subscription: 1 id: -<private info omitted>0035
04-26 18:28:09.657 E/EmergencyNumber( 4193): Found unexpected duplicate numbers: EmergencyNumber:Number-112|CountryIso-|Mnc-|ServiceCategories-0|Urns-[]|Sources-0|Routing-0 vs EmergencyNumber:Number-112|CountryIso-|Mnc-|ServiceCategories-0|Urns-[]|Sources-0|Routing-0
04-26 18:28:09.657 E/EmergencyNumber( 4193): Found unexpected duplicate numbers: EmergencyNumber:Number-911|CountryIso-us|Mnc-|ServiceCategories-111|Urns-[]|Sources-10000|Routing-0 vs EmergencyNumber:Number-911|CountryIso-us|Mnc-|ServiceCategories-111|Urns-[]|Sources-10000|Routing-0
04-26 18:28:09.661 D/ImsSmsDispatcher [1]( 4193): isAvailable: up=true, reg= true, cap= false
04-26 18:28:09.661 D/SubscriptionController( 4193): [getSubId]- numSubIds == 0, return null slotIndex=0
04-26 18:28:09.661 D/SubscriptionController( 4193): [getSlotIndex]- subId invalid
04-26 18:28:09.661 D/EmergencyNumberTracker( 4193): slotId:-1 country:us emergencyNumbers: 911,112,*911,#911
04-26 18:28:09.661 D/EmergencyNumberTracker( 4193): slotId:1 country:us emergencyNumbers: 911,112,*911,#911
04-26 18:28:09.661 E/ImsSmsDispatcher [1]( 4193): Emergency Sms is not supported for: [****]
04-26 18:28:09.661 D/ImsSmsDispatcher [1]( 4193): isAvailable: up=true, reg= true, cap= false
04-26 18:28:09.661 D/ImsSmsDispatcher [1]( 4193): isAvailable: up=true, reg= true, cap= false
04-26 18:28:09.661 D/SMSDispatcher( 4193): sendText id: -<private info omitted>0035
04-26 18:28:09.667 D/SubscriptionController( 4193): [getSubId]- numSubIds == 0, return null slotIndex=0
04-26 18:28:09.667 D/SubscriptionController( 4193): [getSlotIndex]- subId invalid
04-26 18:28:09.667 D/EmergencyNumberTracker( 4193): slotId:-1 country:us emergencyNumbers: 911,112,*911,#911
04-26 18:28:09.667 D/EmergencyNumberTracker( 4193): slotId:1 country:us emergencyNumbers: 911,112,*911,#911
04-26 18:28:09.676 D/SMSDispatcher( 4193): Found carrier package.
04-26 18:28:09.678 D/SMSDispatcher( 4193): bindService() for carrier messaging service succeeded
04-26 18:28:09.686 D/SMSDispatcher( 4193): Sending SMS by IP failed. Retry on carrier network. id: -<private info omitted>0035
04-26 18:28:09.687 D/SubscriptionController( 4193): [getSubId]- numSubIds == 0, return null slotIndex=0
04-26 18:28:09.687 D/SubscriptionController( 4193): [getSlotIndex]- subId invalid
04-26 18:28:09.687 D/EmergencyNumberTracker( 4193): slotId:-1 country:us emergencyNumbers: 911,112,*911,#911
04-26 18:28:09.687 D/EmergencyNumberTracker( 4193): slotId:1 country:us emergencyNumbers: 911,112,*911,#911
04-26 18:28:09.687 D/ImsSmsDispatcher [1]( 4193): isAvailable: up=true, reg= true, cap= false
04-26 18:28:09.687 D/CdmaSMSDispatcher( 4193): sendSms: isIms()=true mRetryCount=0 mImsRetry=0 mMessageRef=0 mUsesImsServiceForIms=false SS=0 id=-<private info omitted>0035
04-26 18:28:09.687 D/ImsSmsDispatcher [1]( 4193): isAvailable: up=true, reg= true, cap= false
04-26 18:28:09.687 D/ImsSmsDispatcher [1]( 4193): isAvailable: up=true, reg= true, cap= false
04-26 18:28:09.689 D/RILJ ( 4193): [1083]> RIL_REQUEST_IMS_SEND_SMS [PHONE1]
04-26 18:28:09.690 D/SubscriptionController( 4193): [getSubId]- numSubIds == 0, return null slotIndex=0
04-26 18:28:09.690 D/SubscriptionController( 4193): [getSlotIndex]- subId invalid
04-26 18:28:09.690 D/EmergencyNumberTracker( 4193): slotId:-1 country:us emergencyNumbers: 911,112,*911,#911
04-26 18:28:09.690 D/EmergencyNumberTracker( 4193): slotId:1 country:us emergencyNumbers: 911,112,*911,#911
04-26 18:28:09.696 D/RILJ ( 4193): [1083]< RIL_REQUEST_IMS_SEND_SMS error 40 [PHONE1]
04-26 18:28:09.697 D/RilRequest( 4193): [1083]< RIL_REQUEST_IMS_SEND_SMS error: com.android.internal.telephony.CommandException: MODEM_ERR ret={ mMessageRef = 0, mErrorCode = -1, mAckPdu = }
04-26 18:28:09.697 D/ImsSmsDispatcher [1]( 4193): isAvailable: up=true, reg= true, cap= false
04-26 18:28:09.702 D/SMSDispatcher( 4193): SMS failed with error 111, errorCode -1

"RIL_REQUEST_IMS_SEND_SMS error 40" seems to be "unexpected modem error"... not very helpful 🤷 https://android.googlesource.com/pl...droid/internal/telephony/RILConstants.java#83