Is it legal to record a phone call without informing the other person?

Recording a conversation you are personally a party to (i.e., you're one of the people on the call) is generally treated differently from illegally intercepting a conversation between two other people you're not part of — courts have, in various matrimonial and civil disputes, permitted recordings of conversations a party was personally involved in to be used as evidence, though privacy considerations remain a genuinely debated and evolving area of law.

That said, using such recordings for purposes beyond your own personal record-keeping — such as circulating them publicly, using them to harass or blackmail the other person, or violating any specific confidentiality obligation you owe them — could expose you to separate legal issues, regardless of whether the initial recording itself was lawful.

If you're recording specifically because you anticipate needing evidence for a legal dispute, it's worth discussing with a lawyer both the likely admissibility of such a recording in your specific type of case, and any privacy-related considerations specific to your situation before relying heavily on this approach.