The dialing is slow and the system ca n ' t ensure the calling party fast and accurately put through the called one who is asked for receiving . 5 . the bell ring is so low that the workers ca n ' t hear in the large - scaled working space with strong noise 它存在的缺点有：工程布线麻烦、费用高；电话线路成本高且利用率低；系统可靠性差；呼叫速度慢，并且不能保证主叫用户快速、准确地找到需要接听的被叫用户；振铃声难以保证在大范围、强噪声的生产场所下的工作人员接听到。
The additional services and telecom services supported by vpmn service include short message service , calling number display ( in the case of intra - network calls , internal short numbers of the calling vpmn group can be displayed ; otherwise the real numbers of the calling users are displayed ) , calling number hiding , call forwarding , call waiting , mobile secretary , ip telephone 支持包括短消息业务、来电显示功能（若是网内呼叫则可设定性地显示主叫的vpmn集团内部短号；其它情况显示主叫的真实号码） 、主叫隐藏、呼叫前转、呼叫等待、移动秘书、 ip电话等在内的多项补充业务和电信业务。
