You provide too little information about your environment. Please at least mention:
- What do your clients want to print?
- What is your setup? Do you have a print server?
- What is your existing network infrastructure?
That said, it seems you would like to remotely print to a printer, the way it is commonly done on a LAN, but over a telephone line instead of a LAN.
This is technically possible, but you'd need a printer that can talk to a telephone line. I'm not aware of any printer that does this. However, you could connect the printer to a router or a computer that has a modem and can dial out via telephone.
So either set up a (small) local server at the client site, or a special router. There used to be routers that made the connection Ethernet <-> analog modem, e.g. the MicroLink 56k Fun LAN from Devolo. No idea if there still is such hardware, but I'd guess there is.