Call waiting

Weird issue I’m having i am running the current version of free pbx on a server and all extensions are remote. The network is a mix of Cisco small business desk phones and analog phones converted via a OBI100 device, also some Linksys ATA adapters. The pbx is behind a linux firewall and all the features of the cisco phones work fine no issues, however on the ata devices when the first call comes in the call is fine until a second call comes in on call waiting, when answered it takes about 10 seconds for any audio to start. Not sure what could be causing this issue I’m assuming a firewall issue somewhere but doesn’t make sense that the desk phones call waiting works fine. If anyone knows any solutions let me know thanks.