I'm having problems.

[cikara sugiyama]

I'm having problems SIP-T20P is disconnected during a call.

A call on your softphone Zoiper is no problem, but the call is cut only SIP-T20P.

I think it's a network problem, but Will there a solution?

SIP-T20P and soft phone has been connected to the router in the same hub.
LAN cable replacement and hub has been replaced.

The latest firmware is "9.72.0.30".

Thank you for your attention.

[CWR]

You don't give us much to work with. Who is your provider?
Zoiper is a third party solution and has nothing to do with Yealink. Softphones generally have an easier time with firewalls and network issues.