We are using the Pretty May Call Center for Skype (version 18.104.22.168)
Which call center version do you use please?
Please make sure you use the latest version which should be v22.214.171.124, you can download it from: http://www.prettymay.net/PMCallCenter-setup.exe
Also, does your server have a sound device?
If not, you can install a VAC, for more details, please refer to below URL:http://www.prettymay.net/callcenter_faq/index.php?pid=knowledgebase&cmd=viewentclient&id=9
Please have a check and let me know if you need more help, my skype ID is calfenzhu, thanks.