Very standard is
I'm sorry. I missed your call.
this is the most standard way of saying this and can be used formally and informally.
If you need to use "couldn't", your sentence should be reformulated to
I'm sorry. I couldn't answer your call.
otherwise, you could say
I'm sorry, I didn't catch your call.
in your expression "answer" is usually used instead of "catch", and "didn't" is usually used instead of "couldn't" the reason for the latter is because you may have to explain your inability to answer the phone, "couldn't" = "not able to".