Communicating with the party

Is there anything like walkie-talkies or comms in the game? Would a Pip-Boy have this function built-in?

I say no, but your game may be different. Much of the tech in Fallout tends to be large and bulky - radio tubes and the like. So I’d be more about the aesthetic than anything else.

I see no reason a Pip boy could not have a radio transmitter. They already have the receiver built in. Not sure about FO4 but in FO76 there ate plenty of places that have what appear to be small base station radios.

You may consider treating the transmitter as a mod to the Pip Boy. Maybe require Science 2, difficulty 3 and unique materials. This could even be handles as a mini quest. Once they have found someone that can do the mod they need to locate a functioning micro transmitter. A program (difficulty 2) loaded on a Pip Boy will detect if any nearby robots have a functioning transmitter. If they manage to disable a robot (not self destruct) it is a difficulty 1 +1 per injury repair test to remove the transmitter.

Also decide if it is an AM or FM radio.
AM will have longer range but subject to interference from weather.
FM is shorter range (a dozen miles at best) but less interference. However it is also line of sight, so inside a building, large buildings between transmitter and receiver or even hills will interfere with the signal.

Also consider power. We can assume the Pip Boy has an internal battery but a transmitter may use a lot more power than normal functions. To keep the radio useful and simple I would say the battery can be charged from a fusion cell. Normal operation could be one charge per month(occasional short message). More frequent use could jump to one charge per day or even 1 per scene. Boosting the signal and/ or extended conversations (1+ hours) may take an extra charge as well. I would set the battery capacity at 4 charges and takes an hour to fully charge.