@chch518 what phone do you have?
For the longest time, PM has been using 3G to transmit voice calls. So, your phone could be set to Automatic and using LTE for data, when calls come, your phone will have to switch over to the 3G network to pickup the calls. This has been like this whether you are on 3G or 4G speed plans
Lately, PM joined other Tier 2 and1 providers and offering to transmit voice on LTE network. You phone will no longer need to switch over to 3G to pickup the calls, as long as the phone is VoLTE compatible on PM network. This will ensure less call drops and better call quality. Also, PM needs make this change to prepare for the sunset of 3G network, which is expected to come end of 2025
