Best I can see here is:
Yealink sends hold (reINVITE) to SBC via TLS/SRTP
SBC sends request to FreePBX via UDP/RTP
FreePBX sends reply back to SBC via UDP/RTP
SBC sends reply to Yealink via UDP/RTP
Yealink is expecting TLS/SRTP and not getting it and sending a 488 back to the SBC/FreePBX.
Is this issue happening on remote devices not using TLS/SRTP? If the Yealink is set to UDP/RTP only does this issue persist?