you most likely need to use custom rules. Can give you a detailed answer if you provide the following info:
1. are you talking about a user extension, an IVR menu or a call queue?
2. What’s the call handling during working hours (i.e. probably call rings, if not answered to voicemail)
3. What’s the call handling after hours (this could be going directly to voicemail, or it could again ring, and if not answered - to voicemail)

Right now, a call comes in and, if not answered, a message plays and it goes to voicemail.  There is no differentiation for work hours or after hours.  I would like to have it play a different message after hours and then go to voicemail.  I hope this answers your questions!!  MANY thanks!

step-by-step for User extensions

1.
Users > click on a given user > User details > Settings & Permissions > Edit Schedule > select Specific schedule instead of 24x7

2. 
While you are looking at User details for this user > scroll down to Phone > Incoming call rules > you’ll see a section for working hours and after hours > click on after hours

 

3. Click on send to voicemail and change it to Ring devices
 

4. configure greeting for after hours
 

step-by-step for call queues
1. Phone System > Groups > Call queues > click on a call queue > Call queue details >  Settings > Schedule > WE ARE NOT GOING TO TOUCH THE SCHEDULE ON THE QUEUE! > Leave it 24x7

2. while looking at the call queue scroll down and click on Call handling and members > Custom rules > Add rule
 

3. Create two custom rules as shown in pics below. In most cases you’d need and it’s better to have 2 custom rules.

3.1

3.2
 

3.3 repeat the same thing but for the PM portion of the week
 

you don’t need to apply this both to users and call queues. Normally you’d apply this to one or the other depending on which is being dialed.