Network Connectivity

Need some help! Just came back from Ireland and ever since I've been back my phone keeps doing this thing where it'll go onto emergency calls only until I turn on and off airplane mode, and that seems to fix it until it does it again, happens a couple times a day but it's quite annoying, anyone know what's happening? I've noticed as well on occasion when I try to call people, even when it says I have perfect signal, it says server unreachable, if anyone knows what's wrong please let me know how to fix it, thanks!