iRule not working

Use a tab infront of your code so it formats properly.

I think I pieced it back together, and it looks like your second if is nested inside your first one. You need to close out of the first if so when it doesn't match, the second if is evaluated

when HTTP_REQUEST { 
    if { [HTTP::host] equals "www.website.com" } { 
        if { [HTTP::path] starts_with "/cell" } { 
            if { [HTTP::uri] contains "client_id" } { 
                log local0. "host=[HTTP::host] path=[HTTP::path] action=allow" 
                log local0. "uri=[HTTP::uri] action=allow"
                return
            }
        }
    }
    if { [HTTP::host] equals "www.website.com" } { 
        if { [HTTP::path] starts_with "/phone/" } { 
            log local0. "host=[HTTP::host] path=[HTTP::path] action=allow" 
            return
        }
    } 
    log local0. "host=[HTTP::host] path=[HTTP::path] action=reject" 
    log local0. "Rejected" 
    reject 
    }
}

I think that should work. It would probably be cleaner to combine your if's together with and statements, and have an if/elseif/else structure.

when HTTP_REQUEST { 
    if { ( [HTTP::host] equals "www.website.com" && [HTTP::path] starts_with "/cell" && [HTTP::uri] contains "client_id" ) } { 
        log local0. "host=[HTTP::host] path=[HTTP::path] action=allow" 
        log local0. "uri=[HTTP::uri] action=allow"
    }
    elseif { ( [HTTP::host] equals "www.website.com" && [HTTP::path] starts_with "/phone/" ) } { 
        log local0. "host=[HTTP::host] path=[HTTP::path] action=allow"
    } else {
    log local0. "host=[HTTP::host] path=[HTTP::path] action=reject" 
    log local0. "Rejected" 
    reject 
    }
}