It's a little bit tough to answer you directly without more info as to what you are trying to achieve.
If you are talking about assigning a static IP address to your phone, on the mobile networks this is not possible AFAIK.
If you are talking about assigning a static IP address to your home WAN, you can talk to your ISP about doing this (they usually charge for it).
Another alternative as someone mentioned is to use a Dynamic IP DNS service, such as No-IP.com
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