There are many mics in your iPhone. The top ones seem to be functioning since you can record video or talk on the speaker.
There is a microphone on the bottom of the phone that could be malfunctioning or simply clogged; that's the one used for placing normal calls.
Look between the charging port and headphones jack port. You'll see a meshed round circle. Try to clean it if it seems clogged.
Otherwise, you need to rule out 2 (simple) potential issues:
- Software: if this issue suddenly started after you updated to iOS 8.4, you can try to back-up your data to iTunes or iCloud, restore your iPhone using iTunes, then restore data and test.
- Hardware step 1: the bottom mic may need replacement. Here's the link to iFixit's step-by-step guide to replacing the lightning connector which also contains the bottom mic. The guide also has links to the needed parts and tools. Once this repair is done, test the phone. If it does not solve your issue, we then need to look at other steps / other possible hardware issues.