Yxz. Because your IT illiterate
The IP addresses change and are unreliable without the proper logs (which the ISPs have and usually only gets seen when the law/police are tying to track someone by IP address). This is because of either/both:
1) The phones are sitting behind a thing called a NAT which means that multiple phones can use the same Internet wide IP address (they will have different IP addresses on the ISP's own network). Therefore anyone outside sees the phones as having the same ip address.
2) The phones change IP addresses as they move around and get turned on and off. IP addresses can move around quite frequently, your phone could change IP addresses daily.
Technical reasoning (backing up what I say): As IPv4 addresses get low companies are NATing boxes and phones are a primary candidate, so phones may be NATed. It is becoming more common for ISPs to use dynamic ip addresses, and with phones the effect will be minimal (as they do not usually run servers).