This issue is on a dual SIM phone.
If on a dual SIM phone the caller has not been contacted from that iPhone earlier, then the SMS response using “Respond with Text” options does not get delivered. The SMS text (not iMessage) message using “Respond with Text” option gets sent and delivered to that caller only if that caller has been contacted from the iPhone earlier (either using SMS or phone call). If that caller has not been contacted earlier from that iPhone, the SMS text message from “Respond with Text” option fails.
This is perhaps because the default SIM to be used for sending that SMS text message does not get selected while using the “Respond with Text” options during a received phone call.
However if that caller is then contacted using either the phone app or using Messages app (text SMS message), the phone line to be used out of two lines) gets selected and then the SMS text message from “Respond with Text” options during a received phone call gets sent and delivered without a problem.
This suggests that the default phone line to be used on a dual SIM phone, for sending SMS text message from “Respond with Text” options during a received phone call is not selected at the back end of Phone app until that caller has been contacted from that iPhone prior to receiving that phone call.
If on a dual SIM phone the caller has not been contacted from that iPhone earlier, then the SMS response using “Respond with Text” options does not get delivered. The SMS text (not iMessage) message using “Respond with Text” option gets sent and delivered to that caller only if that caller has been contacted from the iPhone earlier (either using SMS or phone call). If that caller has not been contacted earlier from that iPhone, the SMS text message from “Respond with Text” option fails.
This is perhaps because the default SIM to be used for sending that SMS text message does not get selected while using the “Respond with Text” options during a received phone call.
However if that caller is then contacted using either the phone app or using Messages app (text SMS message), the phone line to be used out of two lines) gets selected and then the SMS text message from “Respond with Text” options during a received phone call gets sent and delivered without a problem.
This suggests that the default phone line to be used on a dual SIM phone, for sending SMS text message from “Respond with Text” options during a received phone call is not selected at the back end of Phone app until that caller has been contacted from that iPhone prior to receiving that phone call.