Hmm... It is my understanding that it's not uncommon that employers don't pay the AHV contribution to the Ausgleichskasse until the end of the year or the end of January. So from the point of view of the insurer, nothing strange has happened yet and this might explain their "not my problem" attitude. So I propose to leave it at that for now and take action elsewhere. (Was this employer your first in Switzerland, or have you already paid AHV in the past?)

First I recomend you get some legal assistance. That can be a lawyer or another person who is familiar with Swiss law, but there's also another possibility. The lower civil court of canton Basel-Stadt offers 15 minutes counseling to its residents. (Go to an "Auskünfte in allgemeinen zivilrechtlichen Fragen" afternoon [Monday and Friday], you can register starting at 13:15, the sessions begin 14:00).

No matter which option you chose, it is essential that you have all documents ready! This includes the entire written communication between you and the employer: employment contract, salary bill and (fiscal) salary statement if available (you're legally entitled to recieve the latter two), plus any other notes and the proof that you got paid.

The first step you will be advised to make will most probably be to send a registered letter with your demands and an ultimatum to the former employer. These demands include the salary and the missing salary documents... You will need them for AHV and maybe the source tax later.

If the employer fails to comply, possible steps include Betreibung/Poursuite or a labor court case, but that's speculation from my side.