Getting started with query tuning
Seeing the query text, not just the plan, could help us help you.

Your hash joins are resorting to multiple batches, so increasing "work_mem" (by a lot) could help, if you have the RAM to support it.

Increasing "max_parallel_workers_per_gather" could lead to more parallelism which could be beneficial.

An index on (calltime,imsi) if you don't already have one might help, but given that you are fetching nearly a million rows it might not.

What are you going to do with all these rows once you get them?  Maybe you do aggregation on the server, rather than doing it in the client (if that is what you are doing).