Yes, if a vehicle accident is thought to have been caused by a driver who was using their phone when the incident occurred, it may be possible to obtain their phone records. However, this is not an easy process. The injury victim is not going to be able to call the phone company and simply ask for the at-fault party's records.
However, a skilled personal injury lawyer in Montana will be able to help go through the court system to obtain a subpoena for these records. This will likely be necessary if a civil personal injury lawsuit has been filed and it looks like the case could be heading for a trial. The phone records could be obtained and admitted into the case as evidence. These records will be turned over to the other party during the discovery process, and they could help influence the decision about whether or not to settle before a case has to go all the way to trial.