[Serusers] need help badly!!!

Hi All, I cannot call to PSTN, previously I can, so I ask our gateway provider, they are saying nothing's wrong on their network. I'm noticing this on my logs "SIP/2.0 500 I'm terribly sorry, server error occured (2/TM)" it says out of memory, what memory is it? I'm looking on my memory usage it's at 0.01%. also i'm only noticing this on some extensions only, on the others i don't see it. could it be that these problems are the cause why I can't dial to PSTN? But I can still make phone calls when I call local extensions which means my ser is working. How can I contest this to my gateway provider? Hope anyone can help me. Thanks