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When my friend call me on my mobile from overseas , some times I can see his number on my mobile screen (which is save) , some times I can not see the number , I just see call on the screen of the mobile.<br />
<br />
he is using VoIP call from his work<br />
<br />
Why this happens ?<br />
<br />
Regards
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When my friend call me on my mobile from overseas , some times I can see his number on my mobile screen (which is save) , some times I can not see the number , I just see call on the screen of the mobile.

he is using VoIP call from his work

Why this happens ?

Regards

VoIP call from overseas and caller ID

Voice over IP (VoIP) is a methodology and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. Other terms commonly associated with VoIP are IP telephony, Internet telephony, broadband telephony, and broadband phone service. The term specifically refers to the provisioning of communications services (voice, fax, SMS, voice-messaging) over the public Internet, rather than via the public switched telephone network (PSTN).  Examples of the VoIP protocols are H.323, Media Gateway Control Protocol (MGCP), Session Initiation Protocol (SIP), H.248 (also known as Media Gateway Control (Megaco)), Real-time Transport Protocol (RTP), Real-time Transport Control Protocol (RTCP), Secure Real-time Transport Protocol (SRTP), Session Description Protocol (SDP), and Inter-Asterisk eXchange (IAX).