Hi
Im puzzled as first thought is if the Signal in your area is poor generally, you would have this kind of issue. yet you say you have signal...
Or do you find, indoors is when you have trouble... & you only have Edge (2G)?
Sometimes, for reasons unknown, to turn off - pull out battery & sim, buff sim with clean cloth, then re-insert & reboot can help.
Often, you would get a suggestion to check your config settings... this would not apply here - if your settings were incorrect your phone would not work at all.
If your account is fine,(check My 48) & you have no fair use warnings(?), it points to the handset & perhaps poor signal.
Can you try the suggestions for elimination above & post back?
🙂