You might have already done these things, but I'll say them anyways.
CellT, I'm assuming is a variable from a Let calculation, otherwise it needs to be a field, variable, or static text.
The only other thing I could think of was if the phone number field is a number field instead of a text field. I don't think that should matter, though.
Hope that Helps (HTH)
Copied and pasted it into a test calc field and it works fine for me.
What behavior are you seeing?
Some intitial "gotchas":
Is your phone field called "CellT" ?
If you test it with a known 7digit number, does it work then?
Are you trying to use it on repeating fields?
Is your calculation result set to "Text"?
filter did the job - thanks