payload

uint8_t sms_data::payload[160+1]

SMS message payload.

Reserving enough bytes for maximum number of characters but the length of the received payload is in payload_len variable.

Generally the message is of text type in which case you can treat it as string. However, header may contain information that determines it for specific purpose, e.g., via application port information, in which case it should be treated as specified for that purpose.