Port 1723: PPTP VPN
Port 1723 is PPTP, an old VPN protocol whose security is considered broken.
What runs on port 1723
PPTP sets up a VPN tunnel with a control connection on TCP 1723 and carries traffic over GRE. Microsoft popularised it in the 1990s. Its usual MS-CHAPv2 authentication was shown in 2012 to be crackable, and Apple removed PPTP from iOS 10 and macOS Sierra.
On a home network
Some older routers still offer a PPTP VPN server. Modern phones and computers mostly cannot connect to it any more.
If port 1723 is reachable from the internet
A PPTP server on the internet offers a VPN whose traffic and passwords can be recovered by a capable attacker. It is also scanned and brute forced. Replace it with WireGuard, OpenVPN or IKEv2.
How to check whether it is open on your network
Look in your router's VPN server settings for PPTP and turn it off if it is enabled. An outside scan of your public address will show 1723 if it is running.
To find out whether port 1723 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.