Depends on where you are, what phone you have, and exactly how old it really is.
For example:
If you are in the US, using AT&T, then there are all kinds of things that would go against you if your phone is a pre-5G phone.
A year ago, AT&T shut down all 3G networks, and many older phones relied on the 3G (GSM-based) network for regular cellular calls and texts. Even phones that labeled the network as only "4G", without the added "LTE", moniker, we're still using the 3G network.
So those 3G and HSPA+ based 4G phones are now just dead. No service.
If the phone actually worked on the LTE network, then chances of continued use was greatly improved. However, it would need to be approved by AT&T to use on their newer VoLTE cell service for calls. That's what they switched all cell service over to with the 3G shutdown. Not all LTE phones were included. But many were.
With the 3G shutdown, AT&T's plan was to re-farm those bands for some LTE and mostly 5G use.
But, AT&T still seems to be messing around with those towers and trying to figure out the best balance between data-and-voice traffic.
That's pretty much the way with all 3 of the US carriers. With slight differences. T-Mobile seems the most forgiving. As far as other parts of the world, I don't really know. 3G seems be hanging around much longer in other countries. The planned shut down of 3G seems to be a few years away still.