phone signal problem ?

okay so on my galaxy note i have a full signal usually with the H icon, and even with this full signal sometimes my phone gets stuck sending a message or i cannot ring/recieve calls ? it just goes straight through to voicemail. i check my phone and yeah full signal with H. sometimes it works sometimes it doesnt :/

what could be the reasons for this ?

i am in the uk and running the german official jellybean what i got told works fine :)
but also if i remember correctly it did the same thing on a different rom.

any help is much appreciated and thanks you :p