Capturing phone conversations on-screen 10 years ago would require you to patch into the land line that's being used. Not a big deal, but you need some setup time and a land line. Nowadays, in reality anyways, you don't always have the luxury of setup time OR a land line. It'll typically just be phone call from the talent's cell phone, which you obviously can't attach any device onto, unless you have a post department that really enjoys rotoscoping. So instead of fighting speaker phones and awkward phone placement near a lav, just embrace the technology and use it to your advantage.
Stereo mini to TA5 to connect an iPhone to Lectro transmitter: $35
The solution has been in front of you all along. Just take another person's cell phone, preferably the producer's, and take the audio feed out of their headphone jack. From there you can either send to a transmitter or connect it directly to one of your inputs. Chances are, you already have the cables and/or adapters to make it happen. I got Trew Audio to make me a line level stereo mini to TA5 cable to send the feed wireless to me. Our producer wanted to still be able to talk to the person on the other end, so she just kept her phone near her and maintained the controls. Once you have your levels set properly, have the producer call the person that's off camera. This connection will stay active, but the producer will mute their side of the conversation. When the on-screen talent makes the call to the off-screen talent, it'll be just like normal. Just merge the calls to a three-way conversation and BAM, you're in. It works pretty much the same way for making and receiving a call. You get the highest quality sound possible without much intrusion. Just don't forget to put it on it's own track. Location sound is all about problem solving on the fly. If you have the tools and mindset at your disposal, you'll get to lunch much faster.