There may be a more elegant way to do it, but I set up a "global" voicemail box on a dummy extension. You can do a SIP extension, make sure "qualify" is set to yes, and calls will always go to the voicemail box. I've also set up voicemail boxes as a "custom" extension with the dial string CUSTOM/199 (for dummy extension 199).
To get the Message Waiting indicator to flash, do the following (this example assumes the "dummy" voicemail extension is 199 and the receptionist extension is 100):
Open extension 100 (the receptionist's extension) in FreePBX. Under device options, change the "mailbox" line to read:
100@default, 199@default
If the line originally read 100@device, you could use 100@device, 199@device, but I usually set all the mailboxes for default instead.
You can have multiple extensions indicate messages for the "dummy" voicemail boxes. I also set them up to email messages to the appropriate people as well.