A Bad Spell in Yurt by C. Dale Brittain (home page), first book in her Wizard of Yurt series. Here is the Wikipedia synopsis:
This story takes place in the tiny kingdom of Yurt. It reads like a charming, light-hearted story at first, but darker forces soon reveal themselves.
Amongst themselves, the characters refer to the "three that rule the world", the aristocracy, the church, and wizardry. Though the aristocracy do the actual ruling, organized wizardry generally considers itself to be the superior of the three, in part because they put an end to the "Black Wars," wars between kingdoms so violent and bloody that individual wizards were forced to band together to stop them. Churchmen considers themselves superior to wizards, and they are traditional rivals in this semi-medieval world.
The first-person narrator is Daimbert, who has just barely graduated from the wizard's school. He takes up his first post as Royal Wizard of Yurt. Daimbert barely graduated, owing to all that embarrassment with the frogs, yet he has amazing improvisational skills that manage to get him by.
Daimbert soon befriends Joachim, the castle chaplain, attempts to make magical telephones from scratch, learns old herbal magic from his predecessor, fights a dragon from the northern land of wild magic, searches for the source of an evil spell on the king, and is forced to bargain with a demon.
Here, magic is a wild force of four dimensions that is shaped by a wizard's spells or potions, and (usually) is spoken aloud using the Hidden Language, needed to channel magic for the means of a spell. However, wizards can also choose to sell their soul to a demon in return for supernatural powers. The book reads somewhat like a mystery, where Daimbert follows up on many clues throughout the story, eventually suspecting everyone in the castle. The caster of the evil spell is not revealed until the end of the book.
What I remember for certain is that a wizard was hired by a small kingdom to be their court wizard.
I was not a very good wizard. But it was not a very big kingdom. I assumed I was the only person to answer their ad, for in a short time I had a letter back from the king's constable, saying the job was mine if I still wanted it, and that I should report to take up the post of Royal Wizard in six weeks.
His first task was to create a phone for them so they could be connected to the phone network.
"You'll meet the king this evening, but he's authorized me to tell you some of our hopes. We've never had a telephone system, but now that you're here we're sure we'll be able to get one."
I was flabbergasted. In the City telephones were so common that you tended to forget how complicated was the magic by which they ran. It was new magic, too, not more than forty years old, something that Yurt's old wizard would never have learned but which was indeed taught at the wizards' school. How was I going to explain I had managed to avoid that whole sequence of courses?
His phone could make the phone on the other end ring, and he could see the person who answered the phone (this was NOT supposed to be the case, the magic phones were voice only), but neither person could hear the other speak.
Actually, the caller from the defective phone and could both see and hear the callee, but couldn't make himself heard:
Very faintly, from the receiver, I could hear a distant ringing. "Triumph at last!" I thought, but dared say nothing. I held the receiver so Maria could hear as well. She leaned close to me, brushing my cheek.
"Look!" she said with indrawn breath. The glass base of the telephone had lit up. Inside was a miniature but very real scene, a room at the wizards' school, a telephone sitting on a table, and one of the young wizards, one I knew but not well, picking up the receiver.
"Hello!" I cried. "Can you hear me?"
"Hello?" somewhat more dubiously. "Is anyone there?"
The tiny figure inside the telephone base turned his head, as though talking to someone else. "No, I can't hear anyone. It's just silent."
"We're here! We're here! Hello?" I shouted.
"Maybe someone's idea of a joke." We watched his hand move to replace the receiver, and then our telephone went blank.
"We did it!" said Maria, giving me a hard hug that startled me so much that I couldn't answer at once. "We made the telephones work!"
"In fact, we didn't," I said, trying to catch my breath.
Towards the end of the first book, he was able to call for help by repeatedly dialing his friend; his friend did not hear anything, but remembered the partially working phone and realized the mass of 'crank calls' was really the main character trying desperately to make contact.
"So I guess it's all right now," I finished, "even though I'll know, if it ever happens again, to get a demonology expert right away. Someone else, more expert, might have been able to negotiate a settlement with the demon without having to offer it his own life. But what are you doing here? Did the chaplain send you a message?
"No," said Zahlfast, "we got no message, unless that was you calling a month ago. The phone rang at the school, yet there was no one on the line. When I heard about it, at first I just thought someone had called us by mistake, or was doing so for a joke, but then I remembered you and your far-seeing but inaudible telephones."