No, the ability to increase or reduce/lessen the number of rings before a caller is sent to voicemail cannot be changed in Sideline. Sideline works off of your carrier settings, so the amount of rings your carrier has set to default is what will be mirrored in Sideline.
FYI: If you do have weak or no cell service, Sideline may not ring or only ring once before going to voicemail. Again, this is based off your Carrier and is not imposed on our end.
If you have questions, please submit a request.