Caller ID Spoofing: How Scammers Use Local and Trusted Numbers, including alphanumeric names, to Trick You
a little information on caller id spoofing:
spam sms, no number, just name:
They’ve set the originator field to alphanumeric. Among other things, this means it wasn’t sent directly from a cellular device (from where the sender number can’t be modified).
The originator field is 10 bytes maximum for content. When set to alphanumeric, the 7-bit SMS default alphabet is used, limiting the address to 11 characters (10 * 8 / 7 as an integer).
While an Australian mobile number in national format (like 0412345678) will fit as alphanumeric, it won’t when in international format (like +61412345678).
Left as numeric, 20 BCD digits will fit into 10 bytes.
It makes sense for a device to not block specific alphanumeric addresses, because there’s no requirement for them to be unique.