I'm trying to understand how I configure the setting for 10-58. Can someone please explain what i need to do? Is it a required setting & will it help me solve some of my remote IP phone sites, with intermittent issues like, Sip server not found, drooped calls? Keep it simple please. Thanks

Anytime you have IP phones outside of the SV or SL systems subnet you enter the other subnet or subnets in 10-58. This should clear up issues, if not, wireshark will be needed to troubleshoot.
Taking care of 10-58 is a good start.

What IP address should I enter?

The IP address or addresses of the phones outside of the SL or SV systems

Just remember remote sites need to have SIP ALG off as well.

Sip ALG is off, confirmed wit the ISP, they handle the modem/router. port forwarding. I was just looking in 15-05 and I have 3 pairs of extensions that have the same IP.address ie. ext 124 local, has the same IP as the offsite extension 108. I'm assuming .there on the same subnet because the network is default with no IT management.

So if all my off site phones are in the same subnet I don't need to enter anything in 10-58?