Hear external phone ring even it is not ringing

when i dial external number from my sip phone i here early ringing ,even actually the cellphone is not ringing.After few seconds i will here actual phone ring of cell phone .i want to disable this early ring .
thanks in advance…

Asterisk will not generate ringback tone (or tell the upstream device to do so) unless you call Ringing, or it receives a signal from downstream that the phone is ringing, so, if are using Ringing, (or the “r” option in Dial) remove it. Otherwise the issue is outside your control.

Note that there is never any guarantee that ring back is synchronised with actual ringing.