Game in the Brain

Animal Spells create the most sustainable and long term logistics for communications. Animal Handling and Naturalist combined with Beast Soother, Animal Control, and Master can create the foundations for a discipline of messenger birds. This can even go so far as breeding for complete domestication of such. Even when the mage who helped create the breed and techniques is gone its utility in managing much of administration would keep the discipline alive even after several generations. 

Of course the best candidate for such domestication is actually the Raven and Homing Pigeon. If you have magic, then you can prefer the raven because it has less predators. There are other birds of great migratory endurance, like the Bar-Tailed Godwit which has the amazing endurance of 11,000km (6,800miles) that can be bred with a lot of effort for particular purpose. 

In the end you need just One Animal Specialist Mage to get the ball rolling and you would have created a very useful communications network. Merchant would probably get into it, using such to coordinate in their pursuits. 

Special Cyphers and Codes can be used to make communication more efficient and with greater meaning within the text. Specialist cyphers scribes with reading glass (as early as 11C or TL3) can create a large amount of information and detail into a small piece of paper, silk, or cloth. Note that there

Encrypt M135, mind control spell, will be very useful for such purposes.
Self-destruct Messages uses Delay (set some conditions; M131) which has Maintained (M128) with a spell like Ignite. So before reading a secret message, better cast memorize first.  

Many communication spells follow the B241 or M14 Long Distance Modifier, which is x3.3 per increment. In Low Magic setting where it would be rare to have a mage with skill-15 in such distances, you would require ceremonial magic and the generous use of energy paying 20fp to cast a spell at 500% the energy cost to send a small message of 1 minute long 100 miles (160km) without risking critical backfire. In such low magic settings, as long as there is a companion or ability to cast ceremonial magic, the mage can Burn extra FP to offset the difficulty of the casting. 

Communication and Empathy is the Diplomat’s Specialty college. Not only is the diplomat able to influence social occasions, they can read and search minds, hide their intentions, compel truth, and have the ability to report their efforts to their superiors. 

All such spells are better when you know who you are contacting and know where they are. 

Dream Sending M45, count-4 Long-Distance. Needs subject to sleep, and can have miscommunication. 

Dream Projections M46, count-4, Long-Distance. Clearer, but needs the subject to be asleep. 

Mind Sending M47, count-4, Long-Distance. Clear as thought and can be pretty expensive. 

Telepathy M47, count-5, Long-Distance. Two way communication.
Message M174, count-7, Long-Distance. 15 second message per energy spent. 

Communication M48, count-9, Long-Distance. Tele-conferencing even allowing other senses.