Install app linphone from Google Play.
Select "Let's go" -> "I already have a SIP account", fill in the data as follows:
Enter the Username and Password you received when registering (username of the view 3XXX or 1XXXX) and domain: t-pbx.net
Click "Apply" and go to Settings -> Network
STUN server: erase stun.linphone.org, leave the field blank. Transport: TCP, UDP is available. Set UDP.
Congratulations! Now you can call!