Most Lumia phones should also support flip-to-silence (ignoring an incoming call by flipping the phone display down), which you can activate in Settings → Audio → Enhancements.
Microsoft's Gestures Beta app also provides the same functionality, plus a few other handy features. Not sure if it'll work on your Lumia 635, though - Microsoft mentions ...
I looked at your profile and saw you're from the Czech Republic. Microsoft has a page showing the availability for each model per region for new updates. This page can be found here: Availability in Europe.
On this page, I didn't find any available updates for the Nokia Lumia 635.
That seems like a glitch either your phone may have or a bug in the app itself. If it is your phone that is the culprit, then it can be easily fixed by either restarting or soft resetting your phone. A soft reset can be done by holding Volume Up button + Power button at the same time for about 10-15 seconds.
If, however, that didn't fixed your problem, then ...
Your phone is probably missing the APN-Settings for MMS. Check this link for instructions on how to fix this for T-Mobile on Windows Phone 8:
From the start screen, swipe left.
Scroll down to and tap Settings.
Scroll down to and tap Network + wireless.
Scroll down to and tap cellular+SIM.
Scroll down ...