Strange issue when placing callers on hold

So I’m having a very strange issue with when we place callers on hold. A call comes in, the secretary (or anyone else for that matter) puts the caller on hold… when that person is taken OFF of hold, they can NOT hear anything being said…

Any ideas? I can provide whatever extra details are needed.

Thanks in advance!!

Added Details:

Asterisk API
Asterisk CLI
Asterisk Info
Hosted PBX

Cisco SPA504g phones
SIP Trunks

Happens only when a call coming in on a trunk is placed on hold

More details please. At the very minimum we need the version of FreePBX and Asterisk. Distro or build by hand. What type of phones? What type of trunks? I will assume SIP trunks based on the symptoms. Probably a firewall/NAT issue. Does this happen when two phones in your system place a call on hold or just when you place a call that is coming in on a trunk are placed on hold?

Anyone have any ideas? I’ve checked NAT, it’s set to yes on phones and PBX… still having issue…