I would say that the number was that of a public payphone. I guess a cop will get into the payphone and the perp will call.
Although i guess initially the cops were supposed to call the perp who would have been at the pay phone, but i guess something happened (perhaps the sniper saw that it would be very easy for a trap to be sprung on him) and when the cops called he did not answer.
Thus the cops now telling the perp to call them at the number, where a cop would be waiting to answer the call. Pure speculation, but it works out.
The funny thing is i went to watch a movie and i saw this trailer of a sniper who has a person trapped in a public phonebooth! Who knows, maybe this perp is planning to go out in a blaze of glory, but maybe first he wants to shoot a cop in a booth. The perp does seem to be following the media closely, and maybe now he has gone to films. After all think how vulnerable the cop would be. (I am sure some freepers know of which trailer i am talking about ...the one where this guy hears a the phone in the booth ringing, gets inside the booth, and when he picks up the phone a person tells him he has a sniper rifle with a bead on him and if he moves he will shoot him).
However i am certain that if the cops do send one of their own into a phone booth that there will be more police and SWAT snipers around that area to shoot a whole flock of geese! That phonebooth could essentially be the safest place on the globe at that moment!