According to the 2014 Furbearer regs:
RECORDED OR ELECTRONIC DEVICES- It is unlawful to use any recorded or electrically amplified bird or animal sounds or electrically amplified imitations of bird or animal calls or sounds to assist in the hunting, taking, killing or capturing of wildlife, except wolves, predatory animals, and those birds not protected by state and Federal law as per Montana law.
Bobcats are not defined as predatory animals in Montana.
It appears using an ecall to hunt bobcats is in fact illegal.