int gsm_7bit_decode(char *decoded, const uint8_t *user_data, uint8_t length);
int gsm_7bit_encode(uint8_t *result, const char *data);
int gsm_7bit_decode(char *decoded, const uint8_t *user_data, uint8_t length);
int gsm_7bit_encode(uint8_t *result, const char *data);