I suspect that IEEE doesn’t have the “Cited pages” field in the citation template (and as a numbered style, it can get confusing to have cited pages?). You can get around that easily by using the suffix field instead with spaces text and punctuation you might want to appear.
Otherwise you can edit carefully the output style with appropriate “link adjacent text” and “forced separation” characters. I replaced the “link adjacent text” with an asterisk in the example below. these characters in the template will prevent extra spaces or text appearing in those citations without cited pages.
[Bibliography Number|,*p.^pp.*Cited Pages|]