I think it could be a situation ... Suppose UE is attached have default bearer only. Now want to make votle call then first it needs to have QCI 5 dedicated bearer for SIP signalling. For this UE needs to make request to MME to create the dedicated bearer. This will be UE initiated bearer.
After that UE can send SIP msg to IMS server and can send SIP signalling for Volte call. Now IMS server needs to make request for dedicated bearer for volte call to core n/w. This will be the n/w initiated dedicated bearer.