They will try and do the phone call thing, for one it's an easy low effort thing.
If you call back then you're more likely to voluntarily come in to answer a few questions, and they can arrange a convenient time for you. Or it might just be something that can be resolved simply enough to get a lead on something else via a phone call.
Could also be that they don't have enough info to do anything formal and are just fishing and it might get them something. This happened with my brother years ago, when the SCPD found out my brother and a person that they had as a suspect in another "serious crime" had a fist fight a few years previously - apparently another person the SCPD was questioning regarding the crime spree brought up the incident between my brother and the suspect; SCPD was hoping my brother might be able to give SCPD info about their suspect. Unfortunately other than my brother stating he was an asshole and the reason for the fist fight on that one occasion, my brother knew nothing about him. The guy (now suspect in another crime) threw the first punch, my brother won and wasn't hurt so he wasn't interested in pursuing charges, or even getting the police involved. However the police were hoping that my brother would have more info about the guy in general which he couldn't help them on.