I'm struggling to come up with any other sentences that inform the listener at the other end of the phone of the reason for the call; so, any sentence of the form "I'm calling ... [reason]". The main sentence format I'm interested to know is the one with the word "concern". I'd appreciate it if you could educate me on this type of sentences.
I'm calling about a laptop I purchased.
This is probably the most informal way to say it, although it isn't generally considered rude. See (preposition) definition 4.
I'm calling concerning a laptop I purchased.
This is slightly more formal than "about" but fundamentally means the exact same thing.
Your example sentence sounds fine in normal speech too, although it is a fair bit more formal than the other two. It's also more commonly heard as "with regards to" rather than "in regards to", although I've heard both. "Regarding" also works.