In case you use Wifi and the calls have problems such as: not receiving the incoming calls, receiving calls with stuttering, so you should contact with IT to check SIP ALG on the Internet system. Consult the following link to turn off SIP ALG on the modem. https://huongdan.antbuddy.com/kb/huong-dan-tat-sip-alg-tren-cac-modem-pho-thong
In the using process, if there is a wifi-related problem, you can switch your mobile device to use 3G/4G to receive and make calls. Note: you need to turn off wifi completely, only use 3G/4G.
Allow all permissions of application when required to ensure the app works best.