Not previous commenter, but as with other problems in computing, I find that you need to add a level of indirection.

    "CallSessionX" -> participant -> "person A"
    "CallSessionX" -> participant -> "person B"
    "CallSessionX" -> duration -> "10 minute"
    "CallSessionX" -> at_time -> t

Haven't used Sparql or Cypher in many years (used in Neo4j), but with the later you could do a query along the lines

   "person A" <- participant <- call_session -> participant -> "person B" 

To find calls between person A and person B, then you can add a variable binding on the exact node you're interested in (eg call_session) to extract/aggregate attributes.