Springtime: The Bird That Wants To Make A Phone Call

It's definitely spring. The Old Man and I get in the car to go walking .. as you do. A short drive to Penrose Estate and cars are decanting children, dogs, and walkers with sticks. The car park is packed and The Old Man do attract some attention trying to park in the last slot. His frustration at the current car do infuriate and defeat him, causing some outraged stares at an accidental horn beep. We give up and turn round to leave for another car park in Helston, at the lower end of the "trail". That's when I realise that quite some of the sniggering walkers are marshaling their sticks, children and dogs to walk out onto the road again and not into the wooded estate. So it dawns on me that they are parking up here to walk down to Porthleven for the Food Festival (sorry folks, last day today) which by all accounts has been packed out this year

But we manage our walk along the Cober and the woods of Penrose Estate. The first thing I see when I get out of the car ...  are swallows flying above the river. In the woods themselves birds are singing fit to bust: robins, wrens, blackbirds, chiffchaffs. Something more melodious. Blackcap? And then a song I don't know, probably some kind of warbler. I heard once that if you are trying to remember a bird's song then concentrate on catching the rhythmn and make up a phrase that echoes the rhythmn. So I do.
Definitely it's: "I gotta make a phone call, brrrr, brrrr."

I rushed (or hobbled) home and do try out the thing on the internet, starting with warbler songs. Maybe a reed warbler? But I am not convinced. So if anyone knows which bird sings "I gotta make a phone call, brrrr, brrrr." - I'd like help with the answer.