Unable to enter my modem's phone number in MMS carrier settings

In the "SMS and MMS Settings", if I fill in or modify the "MMSC" (or "APN", or "Proxy") field, then press on "Apply", the new information is saved and applied as it should being. This is not what's happening with the "Your modem's phone number" field (which I really need if I believe the closed issue #777 (closed)).

If I filled in that field, pressed "apply", then returned to the "SMS and MMS Settings", that "Your modem's phone number" field remained empty, my modification wasn't taken into account. I should point out that just before pressing "apply" the telephone number appears in blue in the field, and not in red, so I respect the expected format. My version of Chatty is 0.7.3.