Unless it is your only option, you want to avoid using an actual phone line as it will sound like, well, the phone (it cuts some of the clarity out and the bass). If you have people who will only use a phone you can get a skype to phone account (which is currently around $3.00 a month). Other options are to use a zoom.us meeting and have the person call in. The quality will be worse. Another “better than nothing” option is uberconference.com, Here again, you would connect via your microphone and your guest would connect via the phone.
If you are connecting via an interface like the Scarlet 2i2, then send the audio into a portal recorder like the Zoom H1, or H6. This will make it sound better as you are capturing exactly the way you hear it is. With Skype recorders (or its own internal recorder) you lose quality. It's usable, but you ALWAYS want to do what you can to preserve the quality.
If you are on a mac check out Call Recorder. If you are on a PC check out Free Skype Recorder. In both cases, I would find a way to get a local recorder going as well so you have two recordings going.
If you want more flexibility, try Squadcast.fm which records both sides of the conversation, and eliminates those “skype burps” as it records both sides of the podcast locally. If you don't have the need for this, it can also combine the two sides of the conversation into one file.
Currently, in 2019 the easiest and most flexible way to record a phone call is the Rodercaster Pro from Rode. I have and use one of these. This unit does everything. You can connect multiple people via phone, USB, Bluetooth, and directly via a microphone (you could have a total of seven people connected). It's an amazing piece of equipment with a price tag around $600 but it also has a built-in recorder, acts as a mixer, and has built-in audio enhancements.