My thinking is that in the 18th and 19th centuries, magic could do similar things to the inventions made in those centuries in our world, and as such many of those inventions were never made or never became well known. As a result of this the technological growth of this alternate world was stunted, and they're still decades or more away from developing the miniturized electronics required for cell phones (and PCs for that matter) - assuming anyone has thought to research such things at all.
For instance, if they already possessed a magical means of two-way audio & video communication (something which has to be set up in specific locations and cannot easily be moved, like say a pool with runes carved into its stone walls), they might not have invented the electric telegraph or land-line telephones. And if they could light their homes with magic lanterns instead of oil or gas based lanterns, there might not be any incentive to build an electrical grid. And so on.
...It occurs to me this is a lot of speculation built on the facts that we weren't told that Magus took Ashley's phone, and we haven't seen any modern technology in the brief flashbacks we've had to Magus world. I still stand by my claim that my scenario is possible, but knowing Dan it's more likely Magus world has modern tech and either Magus and Sirleck did take Ashley's phone and we don't know it yet, or they simply forgot. (We could even wind up with a scene of "I thought you took her phone!" "No, you were supposed to do that!")