Is it possible to forward calls

I'm wondering if Magicall makes it possible to do the following scenario:
I get a phone call and it's "unknown", I want to forward it to another number but prior to forwarding it I want to change that "unknown" number to another number, a number that I pre-select.
Here's why I would want this feature. I have an online voicemail system I use in place of Tmobile's and when someone calls me and if it gets forwarded to my voicemail it will play a specific greeting based on the phone number. So, for example, when my mom calls my cell phone and I don't answer she gets my voicemail and that voicemail greeting is specific only for my mom. So I'm thinking that if the above scenario is possible I would be able to forward all "unknown" calls to my voicemail and if I can change the number then all calls with that number will have a specific greeting geared for only "unknown" calls. I know I'm asking alot but I figured I'd ask this question here anyway. Thanks.