SIM800L error 601 with JTIoT m2m sim card

A couple of weeks ago, I purchased some m2m sims for JTIoT. I noticed that our devices that use sim800l modules work for a while and then some of them start returning error 601. When I used a sim(MTN) which is not an m2m sim, we didn’t experience such errors and our firmware is written in such a way that it handles errors and retries. Any idea what might be the cause of this?

@Ravi_Pujar Any idea about this?