Sending Codes

Possibly one of the best worst kept secrets in the school of modern linguistics is the concept of 'Sending Codes'. They vary a bit from society to society, organization to organization, even family to family in some cases, so 'eavesdropping' has more than a few limitations with the use of this method but the core concept is nearly ubiquitous in the modern era. The biggest limitation of spells like Message or Sending is the small number of words that can be sent with a single casting, even using more powerful magics or supplemental magic items only gets you so far.
And so code systems were developed, a system where a single number for example 'I'm 114' could convey a complex concept like "I've joined up with a routine caravan escort" while taking up only two "words" within the experienced constraints of the spell. This system takes a bit of brain power or the right reference book to be really proficient in and truly make the most of what it's capable of but it allows people to convey information very quickly and or exceptionally densely, with at least an element of surreptitiousness when needed as someone with no book or the wrong book may get a completely different result translating a spied upon message.

Writing System

The "alphabet" of the Sending Codes system aren't letters (alpha) at all but rather numbers, 0 through 9, combined to make larger digit numbers and fill out the vocabulary.

Vocabulary

The vocabulary is essentially all positive numbers, though above a certain size the numbers start taking longer to say and the efficiency curve decreases noticeably. The numbers each represent common phrases or concepts known by both parties involved in the sending.
Adventuring groups may have codes for the urgency of a situation, common battle strategies, various kinds of threats in order to quickly get the message across.