Q.: "No Call Back Number Rec'd" ?

So: a 20th century Motorola StarTAC on VZW (a Page Plus activation)
receives SMS, occasionally, for which the phone firmware then reports:

+------------------
| No Call Back
| Number Rec'd
+------------------

'Zat an expectable failing on the part of that antique StarTAC (ST-7868-W)?
or on the part of the Page Plus data agreement with VZW?
or on the part of the senders of those SMS?

How to reply to those SMSes, or to learn whence they originated?
(No, no relevant details on the Page Plus web site Call Records page.)

Puzzled, -- tlvp
--
Avant de repondre, jeter la poubelle, SVP.