For instance, I got this message (in PDU) which was sent by "AXIS" with a message "#":
07912638510000230407D0416C720A00000111328193528201 23

07 indicates the length of SMSC information in octets.
91 26 38 51 00 00 23 is the SMSC information (7 octets). Lets just skip this.
04 is the first octet of SMS SUBMIT message. Lets just skip this.
07 is the length of address, means 7 digits contained in the address. (7 digits means 8 characters should be read in the address)
D0 is the type of address and indicates that the type of number is alphanumeric. See: The Type-of-Address octet
41 6C 72 0A is the address.
Lets just skip the rest since they don't make any sense to my problem.

In order to decode the address when type of number is alphanumeric, the real address length (that is 4) is required since it encoded using GSM TS 03.38 7-bit default alphabet.

Unfortunately, we got address length (7) which indicate how many digit contained in the address.

How to get the real address length from address length?
Why to get the real address length from address length should be in that way?

Reference:
SMS messages and the PDU format

Thanks in advance.