Accpet Call button does NOT respond in a webex call on iphone ios 8.1.3

Off late 1 in 5 times I observed that when I have to take Webex call on my iphone6, the accept button for an incoming call does NOT respond. Decline call all though works. I am using Webex on the same phone and trying to call on the same phone.

When I try to take the same call on a different phone - other than the where webex is initiated - all works well.

Need to know if this is iphone issue or some bug in webex in ios8.3