alt Hacker NewsLine simulators are fun. The ports can call each other and there's no need to set any custom init strings on the modem like you need with the 9 volt trick. For some old devices that's a necessity.
You can also use an old VoIP ATA from Linksys/Cisco as a cheap line simulator. Like a fully analog TLS the ports can call each other. They can be a PITA to configure right but they're cheap and work well enough.
I've used all three methods, the TLS is the easiest. An ATA can be useful if you've got more than one and your dial-in server is in a different room from the client you're playing with. An ATA can also be set to "call" another device. So your office ATA can call the basement ATA (with your Linux server) as an example.
It's not particularly custom:
Just issue ATX0D on one modem and ATA on the other. Bingo bango: One modem thinks it's initiating a call (without concern about the lack of dialtone), and the other thinks it is answering a call.
But yeah, simulators are fun. A person I used to call "boss" used one with a dry pair of copper that ran all the way across town for his internet connection for a couple of years.
(His shop had space that was used by a local dialup ISP for a POP. In exchange for the space and the electricity, his shop got free internet over a T1 in the days before we had DSL or DOCSIS. It was a great trade for both parties and I felt giddy downloading things at work at >1.5 Mbps instead of ~0.0336 Mbps.
In an effort to save some money on a dedicated phone line and ISP bill at home, he ordered a cheap circuit from the phone company that they billed as an "alarm circuit" -- it was about $20 per month.
One modem was his at house behaving mostly-normally along with the simulator, and another was at the shop connected to our all-singing, all-dancing 486DX4 Linux box. It was stable and cheap and somewhat ridiculous.)