It’s very important to properly configure SIP communication ports on Zultys Phone systems.
When done right, SIP configuration allows for  proper communication between phone system and user devices : desk phones, softphones and MX Mobile applications.

Configure ports

In MX Admin go to Provision -> SIP and RTP
Below is an image of an example of SIP ports configuration.

UDP Communication port: 5060

Best to use when your Zultys phone system is local and on the same network as your user device or softphone.
This is a standard and “well known” UDP SIP port. Standard SIP aware network devices – firewall and routers will allays treat traffic coming to or from UDP port 5060 as SIP. 
Unfortunately this leads to many problems with SIP aware devices as they will attempt to “help” with communication by sending this traffic through SIP Application Layer gateway (SIP ALG)  but will instead break it. 

Secondary UDP communication port: 6060

Best to use when you want to use UDP protocol but your device or softphone is behind a firewall.
Note: For best results you will have to change source communication port on your device or a softphone to something other than 5060 too, for example 5062
Using secondary UDP port allows to avoid SIP ALG on your router or firewall while UDP keeps number of open connections to minimum. 

TCP communication port: 5060

Best to use if your Internet connection tends to be unstable and you have some packet loss, also behind a firewall.
This will also allow , in most cases, to avoid SIP  ALG on routers and firewalls. Please note that TCP sessions have longer timeouts and this will lead to more connections open on your firewalls.

TLS Communications port: 5061

Best to use: Always.
TLS based communication is encrypted and while also allows you to avoid SIP ALG on routers and firewalls, it’s making your SIP connections secure.  This is essentially a TCP protocol, the encryption of your SIP traffic is a huge advantage

Rest of setting:

Leave at default, in 10+ years working with Zultys phone system I’ve never had to change any of them