Transferred calls will not go to voicemail

[dsears]

Hey guys,
Voicemail works but when anyone transfers a call to another number the voicemail never comes up. If you try to go straight to the voicemail it says the number is busy. Also if we transfer a call to an outside number like a mobile number the cell phone voice will work unless you transfer to a t mobile number. Att, Sprint, and Verizon all work and I have no idea why.